I was perfectly happy with the default/native skin for Windows 7, either with or without its transparency effects.

Like most people I greatly preferred Windows XP in its classic mode because XP's default "luna" skin was so hideous. Whereas the Windows 7 classic mode felt slightly broken and abandoned—new UX behaviours didn't quite line up with expectations set by the traditional interface design. A change of appearance is appropriate when there is a change of behaviour.