I completely agree that UI's something we need to improve on. I do think we've been making progress: We recently redesigned the commit history page, we've cleaned up the repository and account admin pages, and we've been updating the whole site with a more streamlined look and feel.
One of the next big things on our roadmap is redoing the repository header (that thing with all the tabs and buttons). And personally, I'd like to see us standardize on a single repo landing page with everything you need, but that's a little controversial at the moment. :)
We're huge fans of defunkt's pjax library, so expect to see it used more on the site in the future - including in the source browser.
Oh, and we're currently looking for a designer! http://www.atlassian.com/about/careers/listing.jsp?jobID=143