Well this started off just listing the stuff I liked, but I didn't like how short the post was, so I decided to discuss both what I like, and what I dislike. gotta keep up the reputation for walls of text you know!
The good:
BSS - Probably one of the most unifying features of the game. BSS has it's glitches occasionally, but overall the system is quite possibly the single most important thing for keeping the community as centralized as it is. Without a central way to distribute beatmaps some things would be pretty different I think. I just wish you got more slots and had more control over your maps (like the ability to graveyard something at will).
The Editor - The editor for mania is very usable, and makes timing things easier than some other editors for VSRGs. I wouldn't say the editor has any features that set it apart or make it unique, or make it a particularly big asset to mania, but it's at least usable. I know some people will disagree on that one though.
Integrated chat - This feature is probably a big contributor to why I'm still here. Whenever I'm bored of mashing keys to music, I hit one key and can see if any of my friends are online. Usually after a minute or two of chatting, my boredom lifts and I go right back to mashing keys. The ability to communicate directly within the game environment is a very nice feature that also has a lot to do with keeping the community centralized.
The Bad:
The community - Osu's community is incredibly immature. That's not to say that there aren't mature people here, but that the overall impression I get from the community is that the average age must be about 12 (less than half my current age). Mind you, this could be alleviated by allowing for players to have their own private chat rooms, but as far as I know that's still extremely unlikely to happen. The real problem here is the lack of moderation in chat as well as on the forums. This is something I will probably be complaining about for the foreseeable future, because it seems that neither peppy nor anyone else with the power to make changes feels that there's a problem with the current system.
The Editor - Yes, the editor is listed under both of these headings. While the editor is easier to use than some other editors (such as the o2jam note tool, or BMS editors), it doesn't have many particularly useful features for editing mania maps. The horizontal and vertical flip features are handy, but they're holdovers from the standard editor anyway, so the don't really count. The disabling of the "open as reference" feature deserves a mention here as well, because that feature is something I made quite a bit of use of, and I've yet to see an official explanation for why that feature was removed/disabled. I also feel that the editor would benefit greatly from more mania editing specific features, such as better methods for selecting large sections of music, or color coded layering. Hopefully peppy will give the mania editor some love whenever he finally gets around to revamping the mania codebase.
The ranking system - While the intentions behind the ranking system are good (separate the good from the bad) it does a poor job of doing so. The system has alienated members of the community enough to cause them to leave or stop trying to create ranked maps. It also creates an environment where popular mappers/modders receive far more attention than newcomers. The current system is also unfairly reliant on a small number of BATs to bubble and rank maps, which we have seen can be problematic. Beyond all this, the system leads many mappers to focus on meeting the minimum requirements to push for their map to be ranked regardless of its actual quality. Although quality is ultimately subjective, I'm pretty sure that if a mapper didn't really put any thought into the placement of each note, and relies on mods to fix the map, that no matter how many mods they get, the map is likely doomed to be awful, because mods are polish, and it doesn't matter how polished a turd is, it's still a turd. Hopefully modding 2.0 will help promote underexposed mappers and streamline the process of getting ranked in a way that makes things feel less alienating for some mappers.
Autoconverts - I know this is a controversial subject, but I firmly believe that autoconverts have negatively impacted the mania community. Because autoconverts are so drastically different than mania specific maps, it has splintered the mania community into people who support autoconverts, and those who do not. This creates unnecessary animosity between players. While I support the idea that mania should be a place of diversity where maps with various different styles can coexist, I believe that allowing autoconverts to have any effect on a player's ranking has led to this divide. I firmly believe that it was a mistake to allow autoconverts to contribute to PP gain. By allowing autoconverts to contribute to PP and therefore rank, many players are given the opportunity to avoid playing mania specific maps and take the "easy way out" by playing autoconverts.
So, why do I keep playing osu!mania if I have far more bad things to say about it than good?
I've decided to make this community my home for now, and I don't like seeing my home community failing to achieve what I feel is possible. I complain because I care. I want mania to succeed, even in the face of all of these problems, because I see a potential for a game and community even better than what we already have. I'm also not convinced that the communities of other games are really any better, nor are they as accessible or integrated as the community here is.
I don't care about things like rank (although 'm not entirely apathetic to it. I still like to try to keep my rank from dropping too low) or some mostly arbitrary skill tracking system like the BMS Dans. I play osumania because the physical feeling of playing beatmaps is fun, and because when that fails me, there's usually someone there to talk to until I feel like playing again. I could enjoy this feeling in other games, but I chose osu!mania, and I plan on staying here for now.
. Not only that, it is also enjoyable to pass a song that you never thought was possible before. The ranking system is nice too, although there can be a lot said about the current ranked/unranked maps. But overall, there isn't much NOT to like about mania.
For example, A beatmap (example: HD diff) in one mapset have HP drain = 7 and OD = 7. In another case, another beatmap (example: MX diff) in same mapset have HP drain = 8 and OD = 8 or this beatmap is SC diff that have HP drain = 9 and OD = 9. I don't know what other people say, but I think different rate is same different accuracy. But for now, It's not very problem! 
#siapaYangNanya?