Re the python startup time, this is particularly important for smudge/clean filters because git execs the command once per file that's being checked out (for example). I suppose even go/haskell would be a little too slow starting when checking out something like the 100k file repos some git-annex users have. ;)
The one major obvious drawback for it being fixed in git is that it's not backwards compatible with old clients though. Doing it in go is probably a good improvement over the existing solutions of git-media and git-fat but I don't think is the final one.
Funny enough, although I had thought this since I started working with git-fat, I only recently admitted it[1]. Perhaps if I had admitted it when I first started work on it then there's a chance they would have seen it! :-P
[1]https://github.com/cyaninc/git-fat/issues/41#issuecomment-88...
I also looked at git-annex, and I could see using it if it were just me on the project (or as a way of keeping fewer files on my laptop drive), but I was reluctant to add any more complexity to the source control process, since explaining how to use git-annex to the entire team was too big of a barrier.
I have been enjoying the simplicity of git-fat, but running git diff and especially git grep makes me think I should switch to something else.
As for git-lfs relative to git-fat: (1) the Go implementation is probably sensible because Python startup time is very slow, (2) git-lfs needs server-side support so administration and security is more complicated, (3) git-lfs appears to be quite opinionated about when files are transferred and inflated in the working tree. The last point may severely limit ability to work offline/on slow networks and may cause interactive response time to be unacceptable. Some details of the implementation are different and I'd be curious to see performance comparisons among all of our tools.