While I really like SourceTree and it's the first app of its kind that I continue using (I've tried all the others but for some reason or another I dropped them), I really miss how Vim helps with following some important guidelines for commit messages [1][2][3].
On the first line the 50 first chars are highlighted with one color, any char after the 50th column is highlighted in a different one to help you know that it's too long. I tend to write long commit messages, so this is a big help for me.
If you write something on the second line the text is highlighted in a very visible manner (bold orange here). This is not really a big need if you know the guidelines, but it's be nice to have.
From the third line on, the text is automatically wrapped at 72 chars, but I guess a different highlighting similar to the one in the first line would be fine.
Mercurial has a similar guideline for changeset comments [4], so a feature like this would benefit more than just Git users.
I'm not suggesting that you implement exactly the same behaviour that Vim provides, but it would be really nice to have some kind of help on this area.
Cheers!
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[1] http://schacon.github.com/git/user-manual.html#creating-good-commit-messages
[2] http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html
[3] http://gitref.org/basic/#commit
[4] http://mercurial.selenic.com/wiki/ChangeSetComments
