Discoverability. I want to be able to download a library (through whatever means), and then figure out how to use it from within my code. I want to be able to type the name of a class, object, namespace, whatever the language has for that, that corresponds to the name of said library and then just play with it. I want docs for every method and class (or function, object, whatever) that I enter, in a tooltip, at my fingertips. I want autocomplete, not because I am too lazy to type, but because I want to call functions that actually
exist. I want my IDE to tell me which parameters a function expects and what to think about when calling it.
Seriously guys, if I wanted to continuously switch between my editor and a browser with API docs, I'd be using a text-editor. If I wanted to spend 80% of my time going through badly commented undocumented source code written by colleagues or outright (friendly, OSS-contributing) strangers, I'd join a Ruby shop. I want an IDE with discoverability, and a language that supports this well.
My other must-have feature is integrated debugging. An IDE without integrated debugging isn't and IDE, it's a text editor with some plugins.