Nothing, if you don't mind using a centralized development model.
The BSD's tend to be ultra-conservative in regard to version control systems - for example OpenBSD is still on CVS and developed their own version: http://www.opencvs.org
I also find the ability to branch and stash easily two super-useful features. Also, I like not having to go to the server to do diffs, merge, or pull files from other branches. bisect is nice, as well.
I use git-svn at work, so I have everything but easy (svn) branching and I find it super useful.
