Compared to higher diffs, low diffs tend to have simpler and sparser rhythm along with lower and more consistent spacing. After all, those are some of the most straightforward ways to make a map easier.
Rhythm is especially important; with most songs, mapping all the sounds in the song results in a rhythm that's too hard to understand and/or too intense for new players. In other words, you'll most likely need to ignore some of the sounds in the song. It's also a good idea to focus on the most prominent layer in the song so that the map feels as connected to the song as possible. This is a rhythm game, after all. The most prominent layer tends to be the vocals or, in instrumental songs, the melody. If you feel like you need to make the rhythm easier in a part you've already mapped, you can for example map more of the sounds passively (with sliderends and/or repeats), replace a slider with just one circle, or just outright delete an object or two.
"Good difficulty progression" depends on the song and the higher diffs. Generally speaking, though, Normal diffs should introduce players to active (clickable, i.e. circles and sliderheads) half-beat rhythm, while Hard diffs should introduce active 1/4 rhythm. Of course, if the song doesn't have any 1/4, the Hard shouldn't have any either. 1/3 rhythms are usually mapped passively in low diffs unless the song has an entire section with that kind of rhythm.
As for what's good or bad in each diff, you should definitely check out the
difficulty-specific Ranking Criteria. Do note that the guidelines for low diffs listed on the page are practically rules. It's also a good idea to take a good, analytical look at a few ranked low diffs.
