In March 2007, long before Wrath of the Lich King was even announced, I wrote a parody of the announcement, calling the new expansion "The Freezing Jihad". It was basically a comment on how little The Burning Crusade really added to World of Warcraft, and how easy it would be for Blizzard to make lots of "more of the same" expansions. When WotLK was announced, the feature list looked surprisingly similar to my Freezing Jihad parody: Level cap raised to 80, new icy continent, one new class, one new tradeskill, one new battleground. But after actually playing Wrath of the Lich King for 2 weeks now, I am happy to say that WotLK is not a Freezing Jihad. I'm not saying it is perfect, but it is a huge step up from Burning Crusade, and not just a simple extension of the same principles.
The biggest improvement I've seen up to now, compared to Burning Crusade, is the quality of the quests, especially of quest lines. I just read Keen's report on resubscribing to WoW: Keen hadn't played TBC, now made a Death Knight, and immediately noticed the drop in quality when leaving the (WotLK) Death Knight starting zone to enter the (TBC) Hellfire Peninsula. Don't despair, Keen! Experience points needed to level to level 70 have been halved, and once you leave Outlands and enter Northrend you'll notice the jump back up in quest quality level. Not only are quests in Wrath of the Lich King more varied than in Burning Crusade or the original World of Warcraft, they are also better integrated into great quest chains. There is a lot less of random strangers asking you to kill 10 foozles, and a lot more of quests that have a close connection to the lore. And quest chains are now closer together in level, so no longer will you have your quest journal filled up with half-finished quest lines for which you need some more levels to be able to do the end. Plus there are scenes with important lore NPCs at the end of several quest chains. I already met the Lich King three times, just from doing the quests in Howling Fjord, Grizzly Hills, and Zul'Darak.
In Wrath of the Lich King you even gain the power to change the world. That is hard to believe, and of course there is a trick behind it: Phasing. You and the player standing next to you will not necessarily see the same thing before them. For example in Conquest Hold in Grizzly Hills there is a series of quests which ultimately lead to a "regime change" in that place. And depending on whether you did that quest series or not, you will see a different chieftain in the palace. So at least *for you*, the world changes when you take part in a story. Great stuff, and very convincingly done. Different players just might see different phases of the same place.
Wrath of the Lich King also offers a lot more replayability than the Burning Crusade. At least to level 75 you can get by 2 completely different ways, via Howling Fjord and Grizzly Hills, or via Borean Tundra and Dragonblight. I hope I can get from level 75, where I am right now, to 80 in 2 completely different ways too (e.g. Zul'Darak and Storm Peaks vs. Sholazar Basin and Icecrown), but I'm not completely sure of that yet. In any case leveling is relatively fast: I ran CensusPlus and found that there are already more level 80 players than players at any other given level. That still means the majority of players is distributed between 70 and 79, but I guess even I will be 80 before Christmas.
One huge change, already implemented in patch 3.0.2., but more part of WotLK than TBC, is the power level of characters. Everybody is stronger now. I had originally toyed with the idea to retire my Warrior, due to being outdated by the introduction of Death Knights, and suffering from the eternal problem of how to level up with a talent build that is more group-oriented than solo-oriented. But that Warrior in full tank spec and full tank gear is now killing mobs much faster than before, and in my case particularly noticeably faster than my holy Priest, and is so much fun, that I've decided to level the Warrior before the frost Mage. But personal consequences aside, all character classes being a lot more powerful has important consequences to gameplay: You have more choice with whom you want to play. I mentioned a dungeon trip where the tank was a Death Knight with a dps spec and dps gear; and yes, that was harder to heal than a Warrior with tank spec and tank gear. But the important point is that it was *possible* to beat the whole dungeon with that setup. Even simple 5-man dungeons now have more skill checks and less gear checks, making fights more interesting, and doable with a wider range of character classes and talent builds. And apparently that change at least survives into the first raid dungeon, Naxxramas, although that up to now is just hearsay, I haven't visited the place yet. That would have huge social consequences: If "bring the player, not the class" becomes even a partial reality, the very structure of raid guilds and how raids are set up would change for the better.
Nevertheless Wrath of the Lich King also has retained some old flaws. As far as I know raid dungeons still have lockout periods, preventing players from mixing freely on raid nights. In spite of having more zones to level up in, Northrend is still in many places overpopulated, and the same rampant kill-stealing that made Hellfire Peninsula so unpleasant at the start of TBC still continues into the new zones. Now that a significant number of players reached level 77, and re-gained the ability to fly, the flyers ninjaing your ores or quest items while you fight the mobs next to it happens again. But that is more a problem of some players being jerks, and on the positive side of the same coin I've seen multiple situations where players first invite everyone into a group before attacking a named mob, making quest progress faster for everyone.
And in spite of all improvements, Wrath of the Lich King is still World of Warcraft at heart. It is still a game which is very much about gear, and not skill. It is still a PvE game, with some minor PvP elements. It is still a game with comic graphics designed to run well on an old computer, not the latest in high-end graphics effects and photo-realism. Wrath of the Lich King is a very good expansion to World of Warcraft, but it is an expansion, not a fundamental change of the game. Which is good, because fundamental changes of the NGE kind aren't all that appreciate in the end.
So, if you ever liked World of Warcraft, you will probably like Wrath of the Lich King. If you prefer a very different kind of game, WotLK won't change that. And even if you like WotLK, there is realistic chance that you will grow bored of it after a few months. But a few months of fun for under $40 plus $15 per month still sounds like a good deal to me. I have no idea what I'll play in a year, but right now I'm playing this! For me Wrath of the Lich King beats expectations, and that is quite a feat.