I would say the top 4 bland class/specs are:
1. Feral/Guardian Druids, Cat and Bear forms get boring fast. They'd be the most boring class if Resto/Balance didn't save them. The only class/spec combo that I think truly makes me not want to play them because of the boring visuals.
2. All Rogues, odd on this list though because they SHOULD be the most bland. Theoretically you should never see them, not for long at least.
3. All Warriors only because Their armor tends to be flashier. Overall I think they look fine though some of their moves could be made a little more apparent for 3rd person view purposes.
4. Hunters. Marks/BM are the blandest, Survival has some pyro. Traps mitigate some of the appearance blandness and makes it look pretty good overall. Like Rogues you really shouldn't see them much.
5. Everything else. While Dks would probably be in this spot, I think they generally are just as flashy as they need to be.