Earlier today we headed into Naxx for the first time and took down Anub'Rekhan after a few hours at it. The fight seems pretty well designed as the first encounter in the zone. While it's not particularly gear demanding the way, say, the Twin Emperors are, it requires a much higher level of coordination and individual responsibility than anything you'd see until late AQ40. Ultimately it feels a bit easier than the Emperors due to that fight's gear constraints, but it's certainly on par in terms of required skill level.
The thing that impresses me most about Naxxramas though is how interesting they managed to make the trash pulls so far. Hopefully the trash keeps up being difficult without becoming uncontrollable and frustrating as we get further in.