Hunters and Warlocks are going to explode in popularity for obvious reasons. I think Shaman and Paladin on Alliance and Horde respectively will also be popular throughout the expac, more the latter than the former due to Seal of Blood. Apart from that, I think it will stay relatively the same apart from some people more willing to play support classes/specs like SP and Boomkin as they improve a lot compared to Classic.
Just talking about TBC makes me excited, can't wait for it. Really hope they don't mess it up in a standard Blizzard style.