I would honestly advise anyone thinking of starting/re-subbing or re-rolling WoW right now to hang fire until Cataclysm.
As someone who has played since vanilla (initially casually, becoming more hardcore in TBC with fairly high-world ranking T5/6 progression and finishing with unique realm first achievements for a good portion of Wrath) I find it very hard to recommend WoW as a game worth trying in it's current state. Raid content has becoming very bland (and that's ignoring the difficulty arguement) and the risk/reward of gearing characters and alts has become very skewed - Heirlooms and such were designed to make leveling new alts quicker and more attractive, for example, but that is totally overshadowed by the loss of "mandatory" achievements, unique gear/mounts, and particularly realm first titles.
The changes Blizzard have made to the game have done a lot to make more of it accessable to more people (and, as someone who played casually for the last 2 months of their sub) that's certainly a good thing, but I don't think anyone could argue against the fact that this has caused the overall content in the game to become diluted and trivalised.
Cataclysm has the potential to fix a lot of this, and not only introduce new content but also revitalise old content, revamp classes, skill trees and all sorts of technical mechanics for the better. The possibility (thin as it may be) of new realms and new communities is also a major consideration, as all current realms are now fairly well established in themselves and their battlegroups. To my mind, you can't beat a fresh server to forge your MMO identity, but that might just be me!
If you've never played WoW before, by all means go ahead and delve into what is undoubtedly still the most vast, polished and engrossing game available today. But if you're planning to revisit it, and especially if you want to reroll from scratch, then hold off until Cataclysm gives you all the more reason to see Azeroth in a whole new light, because right now there really is a choronic lack of purpose to be had at endgame.