As of writing this, I'm still unsure of Cataclysm, and even a little Skeptical.
I've felt that the game has been getting a little too easy lately, ICC is more fun than hard, the same with RS. I fill most of my time now leveling alts, and messing around with friends, but it's getting old, very slowly but it's getting old. The pain of leveling through Westfall yet again, the only positive is the meeting of Old Blanchy everytime I pass into the zone ^^
The reason I wish to join the Cataclysm Beta is to help me figure out if it's really worth continuing anymore. A new world to explore and level in, new encounters to change the mood of the level range that you're in. THe ability to fly to where you finally want to go without having to follow the roads, without having to be killed the the Cliff that you didn't know was there.
To test out if the changes to talents are good, bad or indifferent, the class changes throwing in a new element to game play, I mean come on, what mages doesn't want summon flaming orbs, or a warlock who isn't impressed by his new sexy green flames :D
When nearing the end though, to see if Blizz have answered everyones prayers, to see if the raids, the heroics, the game in general has become a Challange. Maybe it'll never be Vanilla again, maybe we'll never have to sit in a 40 man raid and hope to God you're the luck one to win the epic chest you've been dreaming of...if you were lucky to even be able to down the boss.
All in all, the main reason I would want to join the Beta would be to try out the new world and all it's features, but probably moreso to make me fall in love with the game the way I did when I first hit login.