The Burning Crusade is off the hook. The new content is amazing!
It starts out simple, Hellfire Peninsula, a transition zone with elements of the old zones and a bit of the new zones. The visuals got a nice bump, more polys, higher res textures, more effects, overall the presentation is of very high quality. The thing that really makes the new content is the design. The zones are bigger, and with quest hubs, mobs have their spawn rated adjusted so you don't have to wait for hours, the new 5-man instances are short, to the point, only a couple of quests, and after the end boss there is a pathway to the exit. Also the wing design of instances are very welcome as a new standard.
The first zone was to be expected, but when I went to Zangarmarsh I was really impressed. You could kill a passive spore mob and gain a 5 minute buff, you could "skin" some of the mobs if you were a herbalist. The instances are under water, and they are challenging. The bosses require more of you than just standing, tanking and dealing damage.
Reputation grinding also seems far less tedious, with more sources of stuff to gain rep from, and gaining much higher rep from turn ins. The rewards also seem attractive.
I'm in Terokkar Forrest at the moment, and this place is visually stunning.