Git AND MERCURIAL client. There's not a single decent Mercurial client on Windows. EasyMercurial is better than nothing, but not by much.
We a few .Net projects, (we mainly do Python), some of the projects we haven't even bothered moving from Subversion, because of the lack of good Windows Mercurial clients.
Having SourceTree on both Mac and Windows is huge to us.
EDIT: Just realized that they didn't include Mercurial support yet. Then I don't care at all.