In addition to those Kanda posted, here are some which remain on my definite todo list:
Upload/download new beatmaps in-game (probably not possible due to legal issues!)
Internet high scores
Special modes (hidden, double/half speed sliders)
Fail screen
Exit screen (options to retry etc.)
Replay mode - ability to save replays
Autoplay mode
Online multiplayer - two players hitting different circles on the same screen
This list used to be a lot longer... but I've been busy
Upload/download new beatmaps in-game (probably not possible due to legal issues!)
Internet high scores
Special modes (hidden, double/half speed sliders)
Fail screen
Exit screen (options to retry etc.)
Replay mode - ability to save replays
Autoplay mode
Online multiplayer - two players hitting different circles on the same screen
This list used to be a lot longer... but I've been busy