IMHO, the only thing they really should do, is let people with newer firmware cache games on the memory stick, but just have a cd check or something on startup to avoid piracy. That thing would solve a lot of problems (loading times, battery life).
If there are problems with the sceen (I sure don't see any), they are presumably fixed (which is why I don't see any). Problems with controls are subjective - I think the D-pad on the DS Lite is far worse than the PSPs, but that doesn't seem to cause any problems on that.