I've been thinking about this for a while:

What a good unique selling point for an online IDE would be, is automated online testing--preferably with continuous integration.

I've been in the position enough times that I'd like to contribute some small fix or feature to a project, only to be put off by the effort necessary to set up a local development environment to make my 5-line change.

An online IDE could completely forego that: fork the code online, make my change, automatically run all the tests (preferably instantly!) so I can check I didn't break anything, and send a pull request!