Watching great players get great plays, achieving something you have spent a lot time practicing, testing yourself and pushing yourself to the limits to achieve those things.
I also enjoy playing well mapped beatmaps with a great song and talking with the community.
Perhaps the fact that i recovered from my dislocated on my arm with osu? Was quite pathetic with the recovery but osu helped. I mean with all those hitorigoto and high score and panda jumps... You'd hope you're not recovering......
i like how Osu gives the user-friendly interface and how it's really easy to edit your Osu (drag & drop new downloaded maps to osu, edit skins, and edit maps.)
My friends suck at osu so at least i can brag to them Also its nice to see yourself gradually improve, it gives the game a lot more of a "skill based" feel
- the mapping (not mapping myself but i really enjoy some maps (in all gamemodes)). they fit the songs so well and it kinda feels nice playing many of them :O - the competetiveness. there's so many things where you can rise. like osu standard, ctb, mania, score ranking, combined leaderboards, pp+, osu!skills, achievements, multiplayer rooms, top 50, etc.
Never get bored playing it, even beating my own personal records is fun, it runs on my crappy laptop on about 300 fps ^_^, and the last but not the least (the best indeed), it's community.