What I'm most looking forward to about Cataclysm? Everything.
In all seriousness, there's not one thing I can pick specifically about this expansion that I like more than any others. Playable goblins and worgen? Awesome. New class combinations like the dwarf shaman, or my personal favorite, the tauren paladin? Even more awesome.
And let's not forget all the changes coming to the world! We'll be seeing everything the Blizzard design team has learned from years of building the world and seeing it applied to the level 1-60 experience, which is simply huge; maybe that grind to Outland won't feel so long once the quests are better balanced, with the excellent design philosophies we've seen from Wrath.
And the little things are important too! Simplifying statistics so that I don't have too constantly min-max my crit for my haste rating? A new secondary profession in Archaeology, and one that looks quite entertaining, especially in comparison to the ones we already have. And of course, we'll be able to fly in Azeroth, which means finally being able to buzz the Stormwind cathedral on the back of my nether drake (assuming it still stands, of course.)
With all the changes coming to WoW with this expansion, what's not to look forward to?
P.S. Fargo has to have designed quests for this expansion, and how amazing will those be?