What seems to be escaping people (or, rather they are unaware) is that the 80+ mobs in the new zones are nothing like the mobs they face until that point. In fact, not too long ago, Blizz doubled (I can't quite recall the exact numbers but it might be more) the damage of these mobs.
You do NOT want the 80+ mobs beating on you. They hit like trucks, and until you get deeper into leveling (82+) you'll be a bit lacking in the Stam department to take alot of hits. Pulling multiple mobs will likely get your pet and/or you killed faster that you'd expect.
With that in mind, I found Destro to be a great 80-85 leveling spec. Aftermath slow and Shadowfury kept mobs at a distance while Backdraft and Conflag output alot of damage very quickly. The mitigation of baseline (and glpyhed) Soul Link and Nether Ward/Prot
was great for when I was taking damage. And, liberal use of Soulburn CD's and Shadowburn to restock shards combined with potent healing from Soul Leech and Soul Harvest kept the downtime very minimal.
Demo was decent, but having weaker than Aff DoTs made chain pulling inefficient; and having slower than Destro nukes made it tedious as well. The Felguard's nice, but doesn't provide a substantial boost to the overall efficiency of the spec.
Aff is nowhere near as powerful with the SL changes. Like, I mentioned, having mobs beating on you is bad, and the ramp up time associated with Shadow's embrace means they'll hit you way more often as Aff than as either of the other 2. Again, combined with poor self-healing, this is not very optimal.
That all being said, play what you like. Each specc has ups and downs. I feel that Destro's up's outweighed those of Demo or Aff.
(Just as a background, I'm really trying to push for some sort of sever 1st, so my focus on the Beta has been levelng and what I can do to maximize it as best as possible. I've pushed each spec from 80 to 85 and these were my findings. Your mileage may vary )