The difference between WotLK and TBC is significant but not all that impressive. It seems that the last ten levels of this new expansion are much easier to gain then it was in the previous. There are far more quests then there were in the first expansion and seems to be focal point of leveling. TBC, I feel, offered more incentive to do instances.
Also, WotLK allows you to do dailies before you’re level 80. TBC only allowed you to rep grind once you were max level. WotLK allows you to do this while you level. One improvement to rep grinding is buying tabards for that faction so that when you do level 80 instances you gain rep with just that faction. This will really cut down on grinding out one instance or set of mobs.
One thing I’m very disappointed in, just as I was with the first expansion, is that gear replacement is minimal if even required. At least in TBC the elite group quests offered you blue items. WotLK doesn’t offer blues for most of the group quests, but instead offers greens. Some of these bosses are quite hard, and the gear is less then adequate.
With TBC the terrain and new mobs were not all that impressive. I personally hated the barren, dry, and desolate feel of the Outlands. I came to really dislike the bleak feel of the Outlands. While Zangarmarsh and Nagrand were prettier, they still had their unlikable aspects. Above all, I couldn’t stand falling off the edge of the world if you went too far. Northrend on the other hand is quite pretty and the new mobs are much more interesting looking. All in all I really enjoyed all of Northrend’s zones and the mobs contained in them.