Ridiculous attention to detail. It isn't the big things, mostly, that draw me to IDEA (though some of them, like the way it handles projects and that Maven Just Works, help a lot), but the thousand little things that someone, somewhere, got right that you don't think about. Unless I think about it, I just notice that the overall experience is somehow more pleasant.

Stuff like what comes first in autocomplete (Eclipse's quality in this regard has been somewhat inconsistent). Refactor suggesting intelligent names for variables based on their types.

And then there's the theming. Normally, Swing apps are somewhat ugly and generally obnoxious. The JetBrains folks have put together a Swing LaF that's beautiful and compact, which also contributes to the overall experience just being pleasant.

I'm happier using IntelliJ.