I'm was evaluating IDEA for a month now, and I can say that I'm truly impressed with it. Of course, there are always some minor things that are maybe easier in some other IDE, but overall, nothing is comparable (I've worked with JBuilder and Eclipse).
The only thing that struck me as a unconvenience are resource requirements. I thought JBuilder was a resource eater, but IDEA seems even bigger. Startup time is quite long, and memory consumption also impressive.
Thus, I just wanted to say that from my point of view, there is generally no major thing to add, but only to improve already existing stuff (bugs, better GUI performance, smaller memory consumption, Plugin API docs etc...).