Compare it to Github's slick UI: https://github.com/blog/760-the-tree-slider
But I am optimistic Bitbucket will change it for the better.
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
I think the competitive advantage for Bitbucket will come from super-enhancing features like the issue tracker. Today, I still have to run Redmine inspite of having Github issues. Give me an industrial quality issue tracker (hooks, assignee features, charting, roles and permissions).
Basically just pull Bugzilla into Bitbucket. Now, that would be a killer feature !
TL;DR : will pay for issues
However, github's UI is really exceptional. Things like the slide-over really make it a smooth experience.
My feedback is that hg is solid, bitbucket guts are solid, and the UI is what's really holding you back (besides the git fanboys hollering really loudly).
See: https://bitbucket.org/ellislab/codeigniter-reactor/
Every major block element on that page except for the very top page navigation has a white background. The project info box has a white background, the branches/tags/etc controls bar has a white background, the commit table has a white background, the footer has a white background - all matching (and failing to stand out from) the page's own white background.
Compare to: https://github.com/rails/rails
The use of varying grays and even a blue background help separate elements from each other, as well as separate them from the page background that they're on top of.
I'm not a designer, but the one thing that has always turned me off from Bitbucket is feeling like I'm looking at a bunch of visual clutter whenever I first look at a page. And it's not because there is a lot of visual clutter (there isn't) but simply because everything on the whole page is on the same white background and, at first glance, just blurs into each other.