Better than ever before. The leveling content is amazing compared to what they offered during Vanilla0, a lot more varied and polished, intergrating story-lines way better with questing content now. Just do the Redridge area for the humans. It's freakin' amazing. Raids and instances offer much more varied content as well, more variation in tactics and style. It's a lot more refined and polished than it was before. I find the storylines a lot more interesting now too, with the new Horde and the tensions that creates throughout Azeroth.
Best thing is though that overal Blizzard has given WoW players a lot more choice on how they want to tackle the game. Just do the questing content? Sure. Want to level with PvPing? They got you covered. Ignore all that and just play group instances with friends? Go right ahead. Just want to see the endgame content? 10-Man raids are for you. Want a real challenge? Go get the 25-man achievements. You can even level without killing stuff if you want.
Radeonx said:
They still aren't nearly as hard as Vanilla was, though.
Then again, boss fights in Vanilla were usually hard because you had to manage 40 people and the spam of monsters. In the expansion boss fights finally were starting to get really interesting tactics. You can do so much more with bossfights when you limit raids to 25 people.