I'm suspecting Cataclysm is going to be a commercially disappointing expansion for WoW. I've been a WoW player since the original, and I'm not getting Cata. In fact, my subscription expires in a couple of days and I have no plans to renew.
Cataclysm has some major issues that are going to interfere with its commercial success. Healing specs are being nerfed very hard, to the point that playing a healer in PvE is not going to be fun for many players, and playing a healer in PvP will be even worse. Expect many players with healing mains to bail out. The PvE content is being made more difficult. While that may make the hardcore fringe happy, for the typical WoW player it only adds frustration. And remember, most of these player never finished the normal mode PvE content in WotLK; even that was too hard for them. The popular random LFD feature is going to be a wasteland.
Tremendous changes, larger than in any previous expansion, are being made to all classes and specs. It is unlikely these will be balanced adequately until well into the expansion, if then.
It doesn't take many defections from your circle of close in-game friends before you begin to question why you're still playing the game. When the stuff really hits the fan in after the next several months, SW:tOR will be coming along to snatch up the disgruntled.
If Cataclysm lives up to its name, I expect ATVI stock to take a considerable hit.
So, since I've been leveling toons from 1 to 65 and 70 at this point during my Beta Testing, I feel entitled to say this: Bullsh!t
I keep hearing these whining comments from people who are fundamentally incapable of adapting - get over yourselves. It's not your game - it's Blizzard's, and they have every right to do what they want.
That said, I've enjoyed Cataclysm immensely - the changes are pretty damn cool to the game world, and it's very interesting to see new takes on old quests and events. The Class changes are going to take some getting used to - I hated playing my toons I copied over or the pre-mades.
However, once I started a new toon from scratch and played through the leveling process and re-learned from the ground up, I was able to go back to those copies and pre-mades and kick ass. It's something I'm going to be recommending to everyone I know who plays - I think it'll help. Don't have to level one from scratch all the way - just 10 levels or so, which only takes about 1.5-2hrs.