Blizzard touched a bit on why they've not used world bosses in Wrath in the recent Twitter developer chat. They observed that there are issues with the bosses that have prevented them from feeling comfortable making more world bosses, such as issues with groups racing to bosses, or the ability of people outside raid groups to deliberately mess up boss attempts. On the other side of the coin, though, they did imply that they like world bosses, and will probably implement more in Cataclysm if and when they can find a satisfactory work-around for the problems.
Now, as a former player of Everquest, where pretty much every major raid boss was a "world boss," I can vouch that it wasn't always pleasant. Druids kiting bosses, uber guilds racing each other to bosses, and arguments over who got to kill what on which week were all common occurrences, and the drama that occurred over such things would put most guild drama in WoW to shame. That said, there's something that's rather epic about fighting a large imposing boss in the "open air," knowing that you're not stuck on a special server and that the fate of the world could be at stake -- or at least, the fate of a poor clueless newbie who could stumble on the guy by accident.
So there's a few questions we could ask here. Are the problems the devs list a good enough reason to keep world bosses off the table? Are they fixable? And either way, what world bosses would you like to see in Cataclysm?
I have to say, I'd sort of like to see that awesome giant Kraken model get used for a boss battle. We've seen it used most in single-person quests, but it's so amazingly large and beautiful that I'd love to see a chance for a raid to take it on, maybe in tandem with the Kvaldir, who are awesome and shouldn't disappear into the mists of "that was last expansion" like the Ethereals mostly did. A Ragnaros-like encounter where the Kvaldir summon the Kraken and attack you every time it "submerges" could be a lot of fun.