A review from a 6 year WoW raiding veteran.
First off, the zones that were hit with the cataclysm are amazing. The new questing zones have a new level of accessibility and story telling that will simply blow you away. If you pick up games for leveling I have to recommend WoW. I can't give Cataclysm any points for this though. All of the areas changed by the cataclysm were made available through a free patch around November. Since they are low level areas, they aren't included in the expansion required content. If you buy only the basic game you get access to the same content without requiring 3 expansions.
This is why I will be focusing on the end game, or at least the 80-85 experience. As most MMO vets will agree with me on this, playing an MMO is all about the end game. Its about the adventures you have when you are already an accomplished hero. The new leveling zones are quite good, fun quests with crazy battles breaking out everywhere. The problem is that unlike the expansions from the past, there is no variety. Burning Crusade was released with 7 new zones. Wrath of the Lich King released with 7 or 8 depending if you count Crystalsong Forest. Cataclysm was released with 5 zones. On top of that, the new content requires you to follow it on the rails since it is an ever changing story. You can't do y until you've done x. There's no freedom in any of the new zones, and creating subsequent alts means you will be repeating the content ad nauseum. That said, the stories are somewhat interesting and depending on your opinion of water areas, the new water zone is quite impressive. However, Ulduum and Twilight Highlands are a lot of copy-pasted material from Tanaris and Hinterlands respectively.
This takes me to the end game. The end game is a disaster. If you strayed away from Wrath of the Lich King because it was too "faceroll" or "easy", I'd ask for your LK hardmode kill, but I know you don't have it. That said, the new environment is ready for you to invade and struggle through. All tiers of content are exclusionary to casual players who aren't interested in being the best hero ever. When you reach end game, you'll find that you have 3 instances at your disposal as a "casual" gamer: Grim Batol, Lost City of the Tol'vir, and Halls of Origination. This is pretty much it as a casual gamer. If you want to try your hand at heroics like everyone could in Wrath of the Lich King, be prepared to be very frustrated, especially as a damage dealing class. Unless you are a tank or a healer, queue times at in excess of 40 minutes, and the fights and content are so unforgiving that unless you are running with a group of fairly serious friends expect the regular content to take several hours, with many deaths and frustrations in between boss attempts. Don't bother complaining or voicing your opinion to the developers, because the self-proclaimed hard core will be more than happy to pounce on you like attack dogs with every insult you can imagine, followed by official posts by the developers with condescending "tips" on playing your class.
Raids have changed too. The blizzard design philosophy has changed to no longer allow "everyone to see content" as referenced by their several press releases during WotLK. The normal mode content is exceedingly punishing and time consuming and after several weeks of attempts my guild of close friends have only been able to down 3 of the 12 normal mode bosses on 10 man. Hard mode content is already live. I have heard from several friends that the 25 man content is substantially easier, but this is mere rumor at this time. The content progression trends seem to affirm this rumor, however.
Next there are the new class mechanics. Going from level 80 to 85 you'll see mana costs increase 5 times, mana only increase 2 times, and healing spells become less than half as effective as they were in the last expansion. Basically, if you're a healer they changed the mechanics from the ground up to make it one of the hardest jobs in the raid/group. Most damage classes saw their complexity compounded, moving from fixed rotations with 5-6 button presses to priority/proc oriented rotations with 7+ buttons. For those with no MMO experience, it means its harder to do your best damage than it was before.
As a last comment, I feel this game was rushed, and the punishing difficulty is designed to keep their multi-million player subscriber base happy while they desperately try and finish the expansion quickly. I think the best Exhibit A is the following: Burning Crusade released with 15 dungeons and added 1. Wrath released with 12 dungeons, and added 4. Cataclysm released with 8, 2 of them were re-releases of old content made level 85.
Bottom line: Not as much content as prior expansions, content significantly more unforgiving to newer players, rude and uncooperative gaming community at large (I've been on 6 different servers, it not just me), little variety in zones, apathetic developing staff, and a big step backwards for mechanics.