Twitter for Mountain Lion

So I've now tried around 4 different Twitter clients for Mac and haven't been entirely satisfied with them. Twitter for Mac is the best IMO, but it hasn't been updated in quite some time and it's starting to show. Anyway, I thought I'd take a stab at what an official Twitter client for Mountain Lion could look like. Still playing around with some concepts, this one's the first.

Still working on the UX, so consider this layout a WIP that is likely to change.

That being said, all feedback is greatly appreciated!