I have three main requests:
An improvement to the rating algorithm. Currently just looking at a problem can bring you down about 100 rating points, if you don't try to solve it. This seems extremely excessive and counterproductive. I think it would be best to either add an option where you can view a problem without necessarily being counted as trying to solve it, and thus not losing rating points, or an improved algorithm, which would only subtract rating points for incorrect answers. I have also noticed that solving a very easy problem (level 1, even), can bring you up as much as 70 points, whereas solving a level 5 problem barely brings you up at all. In fact, if the level 5 problem is solved on the third try, it generally ends up lowering your rating. This seems like a very flawed algorithm that should be revised.
An option to hide problems I've already solved. I've seen this request elsewhere as well, so I won't add much more, but this would be a very convenient option to add.
A return of the points/prizes system. I've also seen this request elsewhere. Whenever I solve a problem I'm told that I've received 100 points, but unfortunately this doesn't mean anything :P Adding the points and prizes again would be a nice incentive.