Mouse control is fine, keyboard control is klunky, but I haven't used either since I discovered my xbox 360 controller works with the game.
Improved support for control pads would be wonderful. My friend's dual shock isn't recognized at all, and I can't jump with ninja mono on my game pad. As it is, left and right triggers are assigned to left and right click, space bar isn't mapped.
The game pad seems to override keyboard controls, I haven't tried two game pads, yet.
The current support of analog sticks works great, at least on the xbox360 controller with XP.
Just checking the length of the song against a known copy would eliminate 90% of cheaters.
In-Game id3 Tag Editing
also, some indication of whether the song is recognized before loading. Perhaps a two-pane navigation panel, songs on the left, and a simplified high scores list on the right.
Grab everything you can, penalty for missed blocks. Make it feel like i'm really playing the song, rather than playing in it.
Implement a lives system, an energy bar, or some way of actually losing a song. The game seems pointless without some incentive to complete a song. There's no sense of victory when you "beat" a song because it's impossible to not finish. In the strictest sense, audiosurf really doesn't even qualify as a 'game' at this point. It's more like a highly interactive music visualization.
The exclamation marks are great, but I'd like to see something more visceral. A rumble, an ongoing siren, a more catching flashing, something so I know when I'm screwing up.
Fill The Void
Give me something to look at before I start playing. The current intro is cool for really twisty tracks, but most aren't. boring.
Individual Scores for Dual Vision
I play dual vision exclusively as cooperative multi-player, it'd be cool to see each side's score at the end, even if only the combined score is submitted to the server.
...with separate parallel tracks for each player, web or single-computer. Powerups to mess with the other player (shuffle their stacks, or score siphons). No need for internet score charts for this mode.
More Elite Modes
Dual Vision Elite and Eraser Elite would work great, Vegas Elite might be a bit loaded unless shuffling was assigned to a click, rather than a shoulder wait. Make the shuffling instantaneous in elite mode, rather than trickling down and I think it would work.