One thing which I always found to be an important, albeit horribly frustrating aspect of osu! is failure. As opposed to the current system where you're simply presented with the classic fail sound, a slowdown, and the fail screen, something more encouraging should show up as well to lessen the degree of ragequits. An existing example of this I have in mind is the Team Fortress 2 death screens, which feature a quick statistic about how you either achieved or exceeded a previous record. In the case of osu!, the furthest a player has gotten into a map (percentage-wise), highest combo on the map, or highest accuracy on the map could be used. If all else fails, again like TF2, having some way of making the fail seem silly or ridiculous (they highlight parts of your character's gibbed body) could also make failures... more friendly. Just a thought to make the game a little less about tryharding and more about having fun.