My review of the WotLK plot so far (with spoilers)
Caveat: I've only played the expansion so far with an alliance character, so everything will be told from that standpoint.
The plot is amazing.
So, there was some complaining about the morally ambiguous plot stuff in the beginning. This isn't saying the plot is bad ... actually, I think it's written pretty well if it's actually making me feel uncomfortable. I haven't really done anything morally ambiguous in several zones, though, and so that has pleased me.
On more plot related points, though ...
The second zone I visited was the Howling Fjord (I mentioned this as having a whole celtic/viking feel that pleased me greatly). The large barbarianesque people you fight there, you find out that they were the progenitors of the human race. Some of them went ahead and had the slightly more evolved children, but this group went about a course of ethnic cleansing. Any "freaks" born were slain along with their parents. You also encounter a new type of dwarf that are made of metal and seem to be very interested in the various Titan artifacts that are still around.
From there, I went to the Dragonblight. There is a lot going on here. The blue dragon flight has, essentially, gone insane. They were the guardians of magic, but their leader was asleep for the last 10k years. He woke up, saw how recklessly magic was being used by the younger races, and decided to try to take that toy away from them. I call them insane because they're essentially raping the ley lines to build the power to crush all other magic users, which is threatening to destroy the planet (due to the concentration of magical energies). The other dragon flights have gathered to stop them, including the black dragons, who have always been your enemies in the past. The dragons, in addition to having to deal with the insanity of the blues, are being beset by the scourge, who are trying to reanimate the bones of the first dragon into a dracolich, which, near as I can tell, would mean they just win. You do some quests to hinder those efforts, as well as other assorted quests for the various dragon flights. My favorite of these being the bronze dragon quests. The bronze dragons are the guardians of time. Lately, they've been fighting against some new dragons that call themselves the infinite dragon flight. The infinites have been trying to change parts of the past (some of them claim to be doing it to forge a better future world, but there's a lot of indication that their true goal is to unravel the timestream with their meddling, thus, destroying everything). The leader of the bronze hasn't been seen in a while, which concerns them a little, but they think that he's off fighting the infinites. There is a lot of indication that he IS the leader of the infinites, which is somewhat backed up by the quest the bronze representative sends you on. You go to the bronze's area, where they're actively fighting waves of infinites, and you set up a device that's suppose to find out who the leader of the infinites is. The device would take time to take it's readings, so you have to protect it from infinites that may try to stop you. When you set the device up, a level 80 version of you shows up to help you fight the infinites (I'm not 80 yet, but I suspect that, when I am, there'll be a quest that sends me back to myself protect the machine). You fend off the evil dragons for long enough that the device finishes and the name it supplies is the name of the leader of the bronze flight. You take this information back to the bronze representative, and they brush it off like the device malfunctioned and spotted their leader, which he takes to be a hopeful sign that the leader is still fighting against the infinites.
That isn't the only awesome happening in the Dragonblight, though. Since it's an active battleground against the Scourge, the alliance and the horde are there fighting against the forces of the lich king. After an extended quest chain, you go to take command of some troops and face down a lich. When you summon the lich, he places you and your troops under a fear effect that makes it so you cannot move. Suddenly, the leader of the alliance forces in the area, a massively powerful human paladin named Highlord Bolvar Fordragon (an NPC that figures very prominently in my favorite old world quest line and I always thought was awesome) shows up and insults the lich a bit. He then notices you and says he's pleased to see you still alive. He then removes the fear effect from you and the troops and says that you and he should fight the lich together. After taking it down, he tells you to finish the mop up and meet him at his forward command. You do so, and he sends you on a few quests (including attempting to gain the aide of the Red Dragon Flight, who tell you that, if they can manage it, they'll send some forces to help you), then says he's going to take his troops to face down the remaining undead in front of the gates to Icecrown (the lich king's area) and asks that you protect the command base. There's then an honest-to-god, Final Fantasy style FMV. The alliance forces go down on to the field of battle. Things start off well, but the Scourge starts to get the upper hand. Then, the horde forces ride into battle on your side, fighting the scourge. Their leader comes face-to-face with Bolvar and says that they couldn't let the alliance have all the fun/honor, and they, together, finish the remaining scourge. The gates open, and the Lich King comes out with more troops. The horde leader rushes him, and the Lich King strikes him down with his run blade, absorbing the horde leader's soul into it. Then, a splinter faction of the forsaken (free undead that normally belong to the horde) launch an chemical/biological weapon onto the battle field, talking about ending all life AND destroying the Lich King (since part of the Lich King is still biological). The weapon seems to injure the Lich King, but he makes it away. Sadly, all of the other troops, Bolvar included, begin dieing. Just then, the leader of the Red Dragon flight (the guardians of life) show up and set fire to the battle field to wipe out the chemical/biological agent and to put a merciful end to the people suffering. You are beckoned down to talk to the dragon, who tells you to take Bolvar's shield back to the king of Stormwind, who was his best friend. You do this, and the king flips out. A powerful human wizard tries to keep the relative peace between the horde and the alliance, but negotiations break down, and you're sent in as part of a strike force to bring the forsaken splinter faction to justice, against the will of the horde. Towards the end, there is a tense scene between the king of Stormwind and the leader of the Horde, but the wizard that was attempting to keep the peace freezes everyone in place and forceably teleports them all back to Stormwind. Any sort of burgeoning peace that had be developing between the Alliance and the Horde has been shattered.
The next zone is much less inspiring. The concepts are still neat, and the zone is gorgeous, but the story isn't nearly as gripping. There's more iron dwarf stuff. There's some more nazi-barbarian stuff. There's some undead stuff.
The zone I just finished was called Zul'Drak. It used to be a giant city of ice trolls. Most of those trolls have succumbed to the will of the Lich King, though two of the anti-scourge factions are there fighting the evil trolls and helping the few remaining non-influenced trolls. As things wind on in this zone, you come to find out that the evil trolls are killing/absorbing their god's and using the power to further the scourge's ends. You rescue one of the god's (a leopard goddess), but you attempts to rescue the others fails (one sacrificed himself in a giant explosion when his worshipers were advancing upon him to kill as many of them as he could). Two of the gods were absorbed. The first that you encounter (essentially Quetzalcoatl) is on the brink of death, holding on by a thread in an underworld of his creation to torture his betrayers). You help him suck his power back out of the prophet that absorbed it, and you kill the prophet. The other one (a rhino god), you free, but you're too late, and the prophet absorbs him. The leopard goddess sends you to collect some items, and you use those items in a ritual to summon the rhino-absorbing prophet. You're fighting him, and he starts a ritual to destroy your group, when the goddess you freed shows up, cancels his rituals, and joins you in the fight. It's a very nice cap to the zone.
A couple of the later dungeons I've done ... I did Halls of Stone last night, where you're in an ancient Titan facility fighting the iron dwarves, and you learn a lot about the origin of the dwarves. I also did the Culling of Stratholme, where you're sent back in time by the Bronze Dragon flight. The Infinite Dragon flight are trying to conceal the fact that Stratholme was plagued when Arthas arrived. It was the city already being enplagued that caused Arthas to cross the line and decide to execute all the citizens of Stratholme. There he is taunted by the supposed creator of the plague, a dreadlord named Mal'Ganis, who taunts his seeming inability to protect his people. This drives Arthas to follow Mal'Ganis to Northrend, take up the cursed runeblade Frostmourne, become a death knight, and ultimately merge with the Lich King, the two becoming a far more powerful being in the merging. You aren't forced to kill any living citizens, but you have to stand by and let Arthas do it to preserve the time line. Both places were very awesome.
All in all, I am very pleased with the expansion.