Hell, if Minecraft has swamps, then they sure didn't make it obvious. Swamps have a different type of tree and foliage growth. Usually accompanied by dark or murky water. I see none of that in minecraft. If there are swamps, then notch should atleast make the area stand out from the rest of the biomes, different color water vegatation, etc. Graphically, there is only a handful of different looks when it comes to biomes.
I'd like a high arctic biome. Not even dirt. Just bare stone covered with snow (and perhaps particularly mountainous with tons of "fiord" like glacial valleys, if possible). I feel like that's the only thing missing as of now.
All of the biomes currently classified seem great to me, but I agree that some could use different foliage or a more obvious effect. Maybe in the future. It's definitely not an integral part of the game, but neither were wolves. When I think of a good game swamp, for example, I think of the Innothule of Everquest, Swamp of Sorrows of WoW, etc. Lots of dark, mossy trees and shallow water between shallow hills.