The bread-and-butter of leveling, questing, is better than ever in Wrath. Questing is more fun from both from a narrative and mechanical perspective.
The narratives are stronger. A scourge-ravaged village where many people have been lost, where you ultimately rally the people to flush the zombies out. Some animal activists work against overzealous animal hunters, you start off helping trapped animals, learning about the hunters, and eventually leading an attack to stop their operation. A gnome encampment trying to learn what happened to their lost people and how the missing are connected to the growing pollution. They're like short stories that you play out in the WoW universe. In original and TBC, you'd have a mix of good story progression with ones that seemed tangential or designed to be filler. A much higher proportion of the quests are tied to these short stories. There's setup, there's a cycle of activity and continuation of story, and finally a conclusion with perhaps a leader to the next chapter. And this story usually ties into the overall theme of the zone, or the neighboring quests.
The mechanics of actually doing a quest are the same–there are collection quests, delivery quests, smack enemies quests, and some others. It's the conditions surrounding these quests that has improved. Most quest hubs have neighboring regions where those quests are performed, and now a quest chain logically progresses within that region with little random leaps to other parts of the world. No more wondering if you're going to be back in the same location for a different quest. My wife and I used to tour the entire zone upon entry, picking up absolutely every quest in the fear that two seemingly unrelated hubs were going to ask us to perform a task in the same location; it's not fun when the Smelly Yeti Extinction Society ask you to kill ten smelly yeti and then two sessions later, the Aromatic Leather Craftymakers ask you to collect ten smelly yeti hides. No more of that, from what I've seen. Now we just come upon a quest hub and do what we like.
Both of these points are subtle, but clearly Blizzard has put a lot of effort into improving the quality of quests. I'm really enjoying them.