Yeah, that's a bit boring. Personally I use the solarized theme, and have invested more time on that, than the default themes. That's also what we use for the screenshots (http://magit.vc/screenshots).

Another reason why the default theme (both in it's light and dark variant) are a bit ugly is that they should work even for users who have customized individual faces. Sticking with gray backgrounds seems like a safe bet.