Most important in my book: an IDE should be a simple batteries included experience. Something tweakable like eclipse or emacs is fine, but I want it tweaked. I don't want to tell my editor where it can find my compiler or what language I'm using. I want to install, type in a syntax highlighted and auto-completed hello world program, and have it run. Plenty of IDE's do this, for example variations of eclipse/netbeans/idea and of course VS.