Introduction
This discussion was done with the mentality of explaining people how and when to be consistent and when to use different patterns, using a variation. Also, I decided to add progression as a way to show how maps should progress as the music gets more intense.

Consistency

Consistency is when there is always a certain behavior performing in a similar way; is a regularity. But... how does consistency apply into osu!?
There are two ways we are going to discuss when it comes to this topic: gameplay-wise and mapping-wise.

Why it is so important to mention gameplay in here? Because people will play your maps, basically... or they should.

1. Gameplay-wise: You are properly able to reproduce every score you have done so far, or even doing something better. You can, although, argue that maps are supposed to be testing your skills to some degree, or that even anxienty of getting a higher score can make you not feel so consistent. I would agree with that... not entirely though, you can still be consistent after retrying the map during a couple of days and keep on getting better and better scores every time, that would make you extremely consistent compared to other results.
   It also does affect mapping. If there is a certain pattern people are struggling with, you should take a closer look into it and see what exactly is causing players not to hit that note, will expand this later on
2. Mapping-wise: Consistency in mapping means that you are repeating the same, or a similar rhythm/pattern/concept/idea/variation/etc., throughout the map. This doesn't necessarily mean you have to copy-paste the rhythm over and over again, but rather that you are able to create something similar to the main idea. As long as the idea is similar and it's not being inconsistent with the rest of the map, it should be fine.

Let's try to expand all of this with some examples!

Starting from 777 SISTERS - Snow in "I love you" by DJPop



What is so 'weird' about that pattern? Problem with this pattern in specific is how bad is introduced into the map. DJPop used a similar rhythm throughout the map but with a different design, all of the other patterns were either more spaced or represented in a more readable way... this pattern isn't the case, it's done in such a way that it's easy for the player to easily misread it. You can argue that "slider are more lenient, so getting a 300 out of this object isn't hard", but that would still be an argument I can't support since it's poorly executed and shouldn't be put into practice!

Another example would be KikuoHana - Nobore! Susume! Takai Tou



This pattern was on the first version of the map. But what makes that pattern so different with the rest of the map? It is considered as a pattern that adds an unnecessary difficulty spike and it's also hard to read. It was also poorly placed at the very end of the map, so missing on that note can be even more frustrating to the players that were planning to full combo the map. It also is poorly implemented because there is no other close pattern to it that indicates there is going to be a stacked note on a stream.

Now the question would be...

can I break with my consistency? Yes, you can. The biggest/easiest example for explaining this is that you are not going to use the same patterns/rhythms on a map that cointains both low and high BPM, it makes sense, it goes along with intensity and progression
Let's suppose you are mapping a song that jumps from 150 BPM to 240 BPM. The patterns you use for any 150 BPM map aren't the same you will use for a 240 BPM map. If you start spamming your cross-screen jumps for the sake of "consistency throughout the map" then you will end up screwing the difficulty by creating a huge spike. This is a perfect example in which you can stop following a certain patterning to create a whole new one for the sake of playability and making sense with the rest of the map.

Problem-solving strategies in osu!

Problem-solving strategies are the steps that one would use to find the problem(s) that are in the way to getting to one's own goal. In this cycle one will recognize the problem, and work around it to find a possible solution to it in order to make it as accurate as possible. In osu!, there are 4 cases of this thing I just mentioned:

1. Lateral thinking: Applies into mapping. You think of a solution through an indirect and creative approach. It does have to make sense, though, being creative doesn't mean you have to map something impossible. You can refer this to Karen's map, for example, creativity is properly executed into something fun to play and technically good
2. Research: Applies into both mapping and gameplay. In mapping, is basically detecting the problem and try to solve it with similar ideas, consistency and variation here are important. Even one little angle can be hard to play due to previous placements.
   On the other hand, while playing, this is done by detecting the problem and studying the pattern in the Editor (this is what Cookiezi does from time to time when he's struggling with a certain pattern, he just studies what is causing him to miss and tries to play that pattern over and over again until he learns how to properly play it).
3. Trial and error: Applies into gameplay. This is the most known, we even hear people talking of this in other videogames, like Dark Souls, for example. You repeat the map/pattern until you learn how to properly play it.
   In Dark Souls (for example) you try to repeat a certain combat until you are properly able to tell what attack is a monster/boss/mob going to do with every single animation
4. Root cause analysis: Applies into both mapping and gameplay. You identify the problem and what is causing it. This is why you always testplay your maps, guys.

Variation

Variation is the act of varying in a certain degree. In osu! this can apply to both music and mapping. Whenever you are listening to any song you may encounter similar pieces of rhythm being interpreted in different ways; a harmony doesn't necessarily have to be consistent throughout the whole song, there are changes in the melody that require that change.... not always, but they might be there, of course.

When mapping, on the other hand, you might find repetitive songs as well, and you may not know how to properly find different patterns for said rhythm. What many people do (including myself) is copying the same rhythm over and over again until the rhythm changes; although that is not the only choice, and definitely not what most people tend to do nowadays.

You can have as many rhythm patterns as you want as long as they keep a similar structure to what you have been mapping. If there are certain beats that are very prominent to be considered as "clickable", then don't map slidertails on them (there are expections, though, like a very long sound-held in the music, like 02:26:268 (1) - on Jenny's The Last Journey Home map)! The "clickable" action is to emphasize an exact point of a song, and rhythm is based around most important sounds. Variation works as long as you are consistent as well, so focus on that too!

A very good example of a map using different rhythm patterns properly executed is beatMARIO - Night of Knights. During the first section of The World you can see how different some rhythm patterns are, alacat tried to follow different instruments without losing the momentum of the song, so the impact on the player isn't lost at any moment. Sometimes the "guitar" or whatever that is, other time the drums, etc. Momentum is built around as the music gets more and more intense. (Refer to Tess' discussion if you want a more detailed explanation)

Variation is always appreciated if it is properly executed. There are many things you should worry about when trying different rhythms, though, such as: intensity of the song; intensity of the map; previous rhythms; vocals; prominent beats (when extending sliders or placing slidertails, for example); etc. It may sound a bit vague but it's kind of hard to explain these things when it is all about what you are mapping to, the song and how you have been mapping till now, you are the one who is supposed to find it out! You can also take a look at kors k&REDALiCE - S2TBTANO*C Anthem feat.Yukacco during the first section of the song, of course.

But... what about pattern variation? is that a thing? yes!Does it work in the exact same way as rhythm? Well, kinda. It's very similar to rhythm, it is all about keeping a similar idea/concept from previous patterns and try to replicate them in one way or another. 'Oh, it doesn't sound hard, should be easy!' Easy there! Just one more thing before you try to execute this: if you are going to introduce something different (similar concept, but still different) from what you have been mapping this far, try to map it in a way that is predictable/readable to the player. Do not introduce an edgy pattern very, very late on the map because it will probably be unexpected. Already talked about this before with Karen's map, so you should know what I am talking of by now. Take care when it comes to variation!

Progression

Progression is a very hard topic in osu! Many people defend it and others just hate it. The most known person to have ever mapped something that truly felt like a huge progression was Fort. Just take a look at his maps, you will notice how different the map gets on every single kiai, especially on cillia - FIRST.

To me, it's okay to increase the spacing as the music gets more and more intense, but you have to take into consideration that it cannot reflect a huge difficulty spike, that would make it just way too exaggerated. The idea of progression is to make it feel that someone who has reached that point into the map is going to have something more challenging, but not exaggerated.

Blue Stahli - Throw Away is one of those maps where intensity is a very important factor. There isn't too much going on in the background music as it's rather calm... 'BUT YUII-, WHY IS THE SR SO HIGH!!!11!'. Drums are very emphasized on the song, so Peachtrees had to find a pattern that suited everything else on the map. Whenever there is a jump pattern, the spacing on the previous sections is getting more and more high in order to tell the player about it. It makes it predictable and "fair" to everyone. 03:30:068 - there is also a very good example of consistency and variation right there, both executed within the same measure.

You also have progression within patterns, like the one in Infected Mushroom - The Legend of the Black Shawarma. There is a 64-notes stream in the middle of the map that is supported by all the other patterns that surrond it; map is built around streams so it is predictable and noticeable. And even after that said pattern, both map and song don't lose any kind of impact.

To sum it up!

1. Using only one unique pattern on a map feels forced and unnatural. If your idea is good, then you should implement it somewhere else, so players don't feel like there's something missing.
2. Increasing the spacing from time to time on a map is okay as long as it makes sense. For example, from Kiai to Kiai, finishing with some more spacing is acceptable as long as it's not forced. Don't turn the map into something exaggerated like in cillia - FIRST by Fort. That just feels forced and unnatural.
3. Every pattern should have enough time from the players to recognize and react to. Although this is where you can disagree the most, it's personal preference I guess. Mapping the same pattern over and over again might feel repetitive but technically speaking, it's the best option as it allows players from telling what type of pattern it is.
4. Game example: why would you implement certain areas on any map just to make your game feel longer? If you don't execute this properly, it might feel boring and empty for the most part, and those things won't be unnoticed.
5. Variation of different rhythms and patterns is okay as long as you maintain a similar concept. For example, introducing only one double at the end of your map for the sake of polarity might not be the best idea, it adds an unnecessary readability spike to the map. Instead, you could try extending the previous object to a slider, or even overmapping specific objects to prevent this fron happening. Using variation is always accepted as long as it's not inconsistent with the rest of the map, that way failing and/or missing that pattern doesn't feel unfair to the player, as it might probably be a poorly introduced pattern. Do not include unnecessary patterns to your map just to make it harder, it is bad!
6. Sometimes, inconsistency will just ruin your map without you noticing it. You don’t think through how a specific pattern would interact with another one. You don’t think of the level of complexity of different mechanics in relation to each other. You don't think whether or not some pattern will work because there was a lack of planning. All you can do to fix this is plan. And playtest afterwords as much as possible just to make sure your planning worked. Make sure everything feels like part of the same thing. Something being completely random compared to your mapping style (or rather, the one you are using on that map) might look stupid.
7. So, is that all? Not just yet! I also want to add a couple of things to what I just said. Planning your map before you even start working on it is usually the way to go, although if that is not your intention, then try not to be switching between different mapping styles. And even if the map would be a Collaboration difficulty, I would still argue that switching from style to style without the player noticing it somehow is not a good idea. In Dark Souls, for example, bosses often change their combat patterns upon entering their "rage mode". You should also find a way of doing that, by either adding a pause inbetween or rather, when the music is totally different from previous measures.