Well, for me it's a thousand tiny things. Some of the macros, the GUI layout in general in Eclipse I find awful. I can't open multiple projects in different windows in Eclipse. I can open multiple projects, but then it's view based system swaps a bunch of things around on me when I switch files. IntelliJ's code inspections seem better. Its popup helpers to fix common issues is nice. Lots of simple usability things like the side bar where I can hover over a hot spot and it'll tell me every implementing subclass of a method or the method I'm implementing and from where.
Whenever I've had to use, or watch someone use, Eclipse it seems like they can do most of the things I can do in IntelliJ, but it takes them longer and it's clunkier.
