Well,if i had to pick the thing that excited me most about Cataclysm it's the idea of how phasing will shape the old world and allow storylines to develop, and show us how what we do will change the enviremont around us. Taking a horde escort quest in ashenvale as an example, beseiging a small night elf stronghold with the intention of slaughtering the guards and claiming the place in the name of the Horde(!) is all well and good, but when you've finished and the guards respawn, it does somewhat put a dampner on things (Said stronghold unironically belongs to the Horde now).
Now we'll actually be able to see what our actions do to the world, or in broader terms, how the world reacts to what we've done, and this to me ups the immersion factor massively. but it also means that a small zone can still have massive amounts of depth and experience in it, despite it's size thanks to it's ever-changing face. It also means certain ambigious things can be resolved. (Like whether or not the Lich King is dead. I'm pretty sure that the gold statue in dalaran phases in when the Lich King is first defeated, i could be wrong but it's what i remember.)
Admittedly it does leave to alot of confusion since different people will be in different phases, but that's not exactly a problem when endgame comes around. (Although there was an issue outside ICC which resulted in two phases of existance i wasn't aware existed, made summoning a right old mess sometimes. My advice, send a warlock.) And i admit if you quest with a friend and they momentarily disappear because they completed a quest moments before you did, that could somewhat blur the whole immersion thing, but at least it allows for some real interactive storytelling. At least now when you kill 10 X and take 7 Y's from Z it might actually mean something.
Although, a Worgen in a tophat is a close second frankly.
Ps. I am aware that blizz arnt gonna use phasing everywhere as much as possible, but it's still an awesome idea and i hope to see it used a lot more than previously.
Pss. I am also aware that it isn't new to Cataclysm, but has certainly been refined, and i can;t wait to see what questing possibilities it'll open up in the expansion
Edited, Aug 2nd 2010 3:06am by Salmaras