The ability to mark a branch "diverged," to prevent (or warn) on attempted merges from master (for example) into a maintenance branch.
The ability to create and track dedicated topic branches, and complementary tooling to encourage rebasing commits in these sorts of branches. We might call them "patch sets" or "sets" rather than "branches." Also, it might be useful to think about using/displaying these commits, when published, in a different way.
Represent merge commits as hyperlinks to the user, when possible. I think GitHub's "network graph" and similar visualizations are great for showing how commits and branches interact and relate to each other.
This would probably require some additional or modifies output from "git log".
Branched stashes (perhaps this is closer to what I'm thinking about for the request regarding topic branches.)
The ability to checkout "working copies," of different points/branches currently from a single repository at the same time, using "native" git utilities.
Related, "shelf" functionality is scriptable, but this too needs to be easier and more well supported.
I think legit is a step in the right direction, but it's weird and probably makes it more difficult to understand what's happening with git conceptually as opposed to the above features which would provide more appropriate conceptual metaphors for the work that would-be-git-users need.