To me, the biggest Bitbucket UI issue isn't the lack of some of Github's snazziness, but the simple lack of visual cues to separate elements on the page.
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.
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.
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.