Ron Perlman as a Klingon.
Ron Perlman rocks.
Jason Isaacs, recently mentioned in the other thread, would make a fine Romulan commander.
Vincent Price would've made a memorable TOS Klingon.
Perlman would have made a great Jem'Hadar/Hirogen too. Ditto Clancy Brown and the late Michael Clarke Duncan.
Vulcans/Romulans - Morena Baccarin, a young Geena Davis, Michael Wincott (fantastic voice). Contemporary James Spader could be a great snarky Vulcan.
Vorta/Ferengi - Alan Cumming, Steve Buscemi, Paul Giamatti (he'd also make an interesting Klingon. Or Neelix).
Cardassians - Chris Eccleston, Tom Hiddleston, William Fincher.