So, my thoughts on Cataclysm:

1. It's Very Linear

It really is. Larísa at the Pink Pigtail Inn compares it to the conveyor belt at the Tower of London. Blink and you miss the cutscene. I cut my MMO teeth on Ultima Online (actually waay before that) which didn't even have quests, so I find this a little strange. I find self-completing quests and hunter pets just appearing violate even my minimal suspension of disbelief. Mind you the world has never been very internally consistent, as James Wallis noted in his seminal study of Azerothian Physics

2. It uses lots of phasing

Finally, the actions you take actually have an effect on the world, despite the fact that there are 13 million other people running around. No more finding Manrik's wife, then watching him send other people off. Kill those nagas, and as long as part of a scripted bit, they stay dead. This is, of course dependent on the Linearity. It would be a nightmare to test all the possible paths through various levels of phasing if people could just do what they wanted.

3. It's very pretty.

Despite being remarkably old, I was sorely tempted to take some serious psychedelic pharmaceuticals when I first hit Vasj'ir. Look at the colours! whee! That's deep man.

WoW has always been more of a game than a sandbox or a virtual world simulation, and it's moving even more in that direction. However, that's not necessarily a problem, since the linearity allows for Story. Finally, your level 80 hero can do heroic level 80 things. You just can't decide what heroic things you want to do.