I'm Impressed. The leveling experience, at least what I've seen of it so far, offers a much more interactive storyline in the sense that what you do actually impacts the world around you. For example, when you defeat the "big bad" the "big bad" isn't still hanging around in the same spot the next time you pass by, the "big bad" is gone! The storyline is still on rails, even more so than before, but the actions you take have a visible impact on the world around you.
I'm not sure yet how exactly Blizzard does this. It would appear in some cases that I'm seeing things in the zone differently from what other people are seeing, while in other cases I may be in a completely different version of the zone. Whatever technique they are using it creates the illusion that my actions are actually impacting on the world, and the transitions are seamless. There are no loading screens after a major change.
So far I've been running with the new goblin race, but my brief experiment with a dwarf shows that the same changes have been applied, at least in part, to the older races as well. This kind of storytelling in a MMOG has been tried before, something similar was done in the Conan MMOG, but it seems to be better integrated here. Once again, Blizzard has taken ideas from other companies, polished them, refined them, and then integrated them into WoW to better the overall experience.
I should point out that only your actions make a difference, not your choices. You have no choice whether or not to kill the "big bad", as if you don't you'll be stuck. It's for that reason that i say that the storyline is even more on rails than before, and I can see where this could be a problem should you choose to level up another character of the same race as there will be even less variation in the paths you can choose. A common tactic in the past would be to switch to a different starting area entirely, and I'm not sure that is an option any more.
Another potential problem is that a bugged quest is more critical. If you can't get past a storyline quest you're stuck. I nearly ran into this once with my goblin where I had to defeat a "big bad" that was acting as if it were stuck whenever I attacked from range and that my pet wouldn't attack in melee. Since I'm playing a hunter, my inherent melee attack wasn't enough to defeat the "big bad" and I was stuck until the "big bad" partially reset and my pet at least decided to join in (although I still couldn't attack from range).
We'll just have to wait and see what, if any, effects these problems present down the road. In the meantime I'm having a blast with the game.
Will this cause me to return to WoW with the same level of commitment I showed to the original launch where I played for months and was one of the first to hit the original level cap of 60? Probably not. At this point MMOGs are really just another game for me. I run through all or a portion of the leveling content and then end my subscription since I've never had any interest in the endgame content of any MMOG with the possible exception of the old Star Wars Galaxy MMOG. I no longer buy anything but month to month subscriptions because I know that I'm unlikely to still be playing three months after release, and so far I haven't seen anything about Cataclysm that would change that pattern.