I want the mountains because they are updating the snow mechanics, that's kinda really cool, because there's bound to be a lot more that we'll be able to do with it. Heck, I'm thinking there will be more redstone usage for it.
Also I would love a caves update. I'm personally a miner when it comes down to it, I will just mine for hours on end, so having more challenges and new stuff down here would be nice. I honestly spend more time underground than above ground, so I would really like some new stuff for the caves.
In the mean time I'll be a dwarf digging a hole, diggy diggy hole.