Slightly longer summary: Really delighted.
Let me break out some specific topics.
• Presentation. There's more graphical detail, and it's outstanding. For a great demonstration of this, check out Andromache's post on the subject and be sure to click the picture of his dwarf to get the larger version. There's also no problem I've yet seen with clashing colors and patterns that led some people to describe early Outland gear as all being "of the assclown" in addition to any other properties it had. Early Northrend gear is subdued but brighter colors come in as one goes along, making for a pleasing progression. The music is also a pleasure, very rich and enjoyable—I've got the game music on a lot more than I never did before.
When Tivara first landed in Borean Tundra, I really did gasp a time or two while looking at the scenery. Then came the Howling Fjord, and I thought, "Wow, this is just amazing." Then I saw Utgarde Keep and thought, "This is just really amazing, one of the best-looking instances ever." Then I saw the Nexus and thought, "They've outdone themselves, this is maybe the best-looking instance ever." Wyrmrest Temple and environs: "Astounding." Azjol'Nerub: "This is brilliant. I have never seen so utterly breathtaking, amazing an instance. How can they top this?" Dunno yet, but I'm looking forward to finding out. :)
• Participation. Be it noted that I have a sometimes-pathological loathing for a lot of jargon, and am prepared to go some distance to avoid many popular buzzwords. "Interactive" has gotten my goat recently, since I think it folds together concepts that deserve separate consideration. WoW hasn't gotten any more interactive in the sense that you do things other than pick up quests, perform their tasks, turn them in, and so on. What has changed is the kinds of situations that Blizzard presents for players and their characters, the range of available responses, and the reactions that our choices provoke from bystanders of all sorts.
The nature of the story has changed. Any player character who's made it to Northrend is presumed to have what it takes...by most NPCs, but not by all. Some are always going to be scornful, while others can be won over. But most authorities are prepared to grant some respect to someone who's come through everything PCs have to get that far, and a lot of folks are outright grateful for help. Furthermore, characters aren't just left to their own devices. There's a lot of tool use, from specialized implements of torture to tanks to drive and dragons to fly upon. Characters often join in battles already underway, and contribute to their success (or failure), and summon aid, and rescue trapped comrades, and so on.
• Activity. WoW has always had some fun stuff going on that doesn't depend on the characters, and indeed much that can be seen only when characters can avoid triggering hostilities. There are herds of gazelles in the Barrens, and skeletal guards who throw some of their own bones for their demonic hounds to chase, and on and on. But the expansion pack drives the level of independent action way up. Many more NPCs move around, rather than just standing or sitting in one place. This evening, Tivara fought some trolls on the eastern coast of Northrend, and the ones at water level were fishing rather than patrolling. More predatory animals hunt prey; more prey dodges and fights back. It's an even more vivid world to just sit and watch sometimes.
• Plots and Phasing. Plotting is hard in an MMO environment—world-changing payoffs would break or at least drastically change the game for everyone who comes along later. One of the things that make Blizzard such a success is that it learns from others. Other MMOs introduced various ways to make significant parts of the world behave like the dungeons and other features inside instanced areas. Blizzard's version is called phasing, and what it does is this: it changes the environment and NPCs shown to you in a particular area depending on your character's status. In the death knight starting quest sequences, for instances, first you see the town of New Avalon with blue skies and green fields and the assault just beginning; after winning some milestone victories, you see the northern reaches burned and sacked but the southern ones still intact; finally you see it all in flames and ruins.
(Yes, of course I have screen shots, but this is a text post. Stop sniggering like that, I can too write with nothing more visual than the occasional emoticon.)
They use the same technique in Northrend to dazzling effect at least once. Quests all over the Dragonblight in the mid-70s level range lead to a cut-scene movie of a particularly epic battle and then use phasing to let those who've seen the fight see its aftermath, on the spot and elsewhere. (I'm being vague because it's really, really, really worth getting to see the events unspoiled.) Your character spends an hour or more in customized versions of familiar places as well as the battlefield, with strategic consequences. And even though the phasing ends and you resume seeing the regular versions, the things that happen matter to future quests.
• Continuity and Visibility. The storylines in Outland weren't as disconnected from developments back home on Azeroth as they often seemed, but the connections were often buried deep, and there's a lot that I personally never saw because they were made manifest in play only deep inside the endgame raid instances. Blizzard's people paid attention to the complaints about this. Now the action is up front and accessible, and there are a lot of connections both big and small to what's come before. There are a lot of faces that'll be familiar to players, if they paid attention early on—I was profoundly pleased to find Gryan Stoutmantle running an Alliance outpost in the Grizzly Hills, years after my level 10-20 Alliance characters helped his force clear out bandits in Westfall, and like that. And these people have stories, too, with their own advancing careers and agendas, families and friends to be concerned about, and all the rest. So the whole enterprise really feels like ongoing developments in places and with people we have a prior history with.
All I need now is to find Mankrik and my joy will be complete. :)
• Upgrades. The curve of niftiness was really too steep in Burning Crusade—it's fun to get new things, but it's hard when gear you put in serious play time and effort to acquire for your character becomes useless in, literally, minutes into the first zone of the newly accessible territory. The upgrading in Lich King, on the other hand, has been very satisfying. As Tivara closes in on the last few bars to 76, she's ditched about half the gear she came to Northrend with. This feels right, partly because it's pretty much all been rewards for completing whole chains of quests or major single-quest achievements.
• High-End Craft Learning. There's a really nifty new mechanism for acquiring high-end recipes for cooking, patterns for leatherworking, and so on. (I assume this applies to the other professions too, but these are the only ones I have direct experience.) Past skill level 400 (out of a currently possible 450), you don't just go out and buy new recipes. For leatherworking, you trade in sets of heavy borean leather or arctic fur; for cooking, you perform daily cooking quests—fixing up dishes with a couple recipes or a recipe and some gathered extras and delivering them to commissioning clients spread across Dalaran. This gives you a cooking commendation award token and a bag of rewards including spices you'll use in future cooking, and sets of tokens let you buy recipes. So progression in the craft hinges on actually using it more. This is a relatively small thing overall, but it really caught my fancy and felt significant.
And, no doubt, more will come to mind as soon as I press the Publish button.