Well, I'd say that they did a bad job with Naxx in two ways:
1. It's EXACTLY the same as old Naxx. Had they modified it for WOTLK to reflect Arthas more, it would have been a lot better.
2. It's too easy. Even for casual raiders, it's too easy.
I think that this was a result of the focus being on creating an entirely new continent, creating the new DK area in EPL (an existing content area) and the balancing of an entire new class. The EPL modification adds probably the best starting area in the game, and was one of the highlights for me in WOTLK.
Now, with this expac, I think (hope?) you'll see a lot more attention to the details and concentration on creating exciting and fun content within existing boundaries.
I do agree though with Krunkle, that if it turns out to be WC with L82 mobs, it will be horrible. But if they make it into a world changing, phase based difference, it will be really cool. Like, you can still level through the areas, but then when you hit 80, they start changing with the storyline. I think there's a lot of potential here.