I think community features are a big area that Bitbucket is still growing into -- individual projects/repos are quite nicely featured these days, one just doesn't really have many ways to find them unless you're linked from somewhere or notice them through a user you're following.
This would be a nifty area to start -- not just subjective "notable" projects, but automated metrics for projects with the most followers, the most activity in a recent period, project language analysis, etc. I just noticed that Github does most of this after thinking of all of it myself, so I guess they're natural things to arrive at.
On that note and in the subjective area, they do a pretty nice job of featuring interesting projects on their blog -- just requires editorial time and effort, certainly. Integrated blog that uses my site auth for commenting is nice.
I'm sure you guys are already thinking about a lot this stuff :-)