Notifications/retrieval timing (instant), starring, labels and possibly deletion handling (at least if using Gmail). I think because it started as a Gmail frontend most of its defaults (and there aren't many setting adjustments) were created with that service in mind. Reportedly works well with other IMAPs too, but all my accounts are there.
EDIT: Here's an out of date review: http://www.geek.com/...itter-20110211/
And an old vid:
http://www.youtube.com/watch?v=8KzCHZcpyfg