The reason Wrath has been more fun to play than the Burning Crusade really comes down to one thing: Story.
And I don't mean "story" as in "lore", although the tale that Blizzard has woven as a framework for these games, borrowing heavily from every other fantasy setting ever invented, is a big part of it.
When I say "story" is the key to why questing in Wrath is more fun and the environments are more exciting, I'm talking about a larger and more abstract concept. It's what Blizzard Lead World Designer Alex Afrasiabi talked about in a recent interview on 1up's Legendary Thread: trying to inject story into every aspect of Northrend.
Every inch of Northrend has context. Every area has a reason for being, and every quest has a purpose that jives with that reason. They actually attempt to be logically coherent. The zones are interesting because when you stumble upon a new area, you immediately get a sense that there is something already going on here. The zone is a lot closer to being alive. This is not a simple village of sinister trolls standing still, waiting to be slaughtered by you. This is a group of living people who were already doing something (likely sinister) before you came along to disrupt their plans (likely by slaughtering them). The quest hub isn't just randomly placed because you needed some quests. It's there because it's holding back an attacking Necropolis, or is a long-standing camp of woodsmen, or is a recently-awakened base for ancient creations of the Titans.
Think back to Nagrand. Sure it was beautiful. But what was actually there? A town for your faction. For alliance, it had no story, and no reason for being there. For horde, it served as a thin context for you to discover Thrall's grandmother. Aside from that, there were just a bunch of Ogre towns and animals. Nothing was really going on, and there was no purpose behind any of it. It was only there because they needed someplace for you to quest at that level, and you only went there because, well, how else were you going to hit 70? The only really memorable area, with a feeling of backstory and "this is different", is the big stone in the southwest. It turns out to be the resting place of a Naaru in the dark phase of its lifecycle, where it must absorb souls in order to "relight" itself back to the standard Naaru state. This is some pretty cool backstory and purpose. And it was sorely lacking in the rest of the game. Do you have any idea WHY area 52 is where it is? Do you even remember anything about Blade's Edge? Thought not.
Shadowmoon and Hellfire, two areas rich with Warcraft 1 and 2 backstory, were better, but still failed in general to really give the player the sense that things were going on worth getting involved in.
Though far from perfect on this (or any) front, Wrath has "story" in spades. Unlike in the Burning Crusade, almost nothing is arbitrary. Here are some of my favorite examples:
- In the zone Zul'Drak, you're sent to a series of Troll temples where a tribe is imprisoning some of the Northrend animal aspects (like the aspects of Zul'Gurub and Zul'Aman). At one point, the bear, lynx, eagle, and dragonhawk spirits from ZA actually aid you in attempting to free the imprisoned troll demigods.
- Nearby in Zul'Drak is an Argent Dawn camp pushed to the point of desperation. Their base in ruins, and beset on all sides by undead, they fight on, imploring you to go out there and bring in some of their wounded. It's the aftermath of a great battle in which the Argent Dawn actually claimed victory by bringing down a floating Necropolis from the sky. They also send you to finish off the inhabitants of the ruined Necropolis.
- In Grizzly Hills, Furbolgs have taken up residence in a fallen world tree, and it is up to you to find the source of the corruption around the zone before they can bring a corrupted demigod back to life.
- Sholazar Basin is home to an ongoing war between two native factions, either of which you may choose to aid. Meanwhile, Nesingwary hunts new game. Though Nesingwary is nothing new, the big difference in this zone is the clearly defined geography. Though they share the jungle theme, there are many very different areas, each with their own purpose for being different, whether that be scourge attack or Titan artifacts.
As Tobold has observed, questing was originally conceived as a way of spreading out the player base and getting them to visit different areas of the zone and try killing different targets. With this iteration, the concept has finally eveolved to the point where, instead of the xp and items/cash being the carrot that forces you to explore when you'd rather just sit in one place and grind, they now act more like a tour guide where seeing something new and learning more about the story is actually part of the carrot.
They've finally succeeded at creating a game world as a theme park.