Elader Arkon
Helped Tek test a thing
First Point: Phasing has been being used in online games for over 10 years by many different companies...and the reason I call GWs lobby phased, because sometimes you are in the same lobby as your friends and sometimes you aren't, but you are in the same area...
Third and Fourth: Actually playing on Xbox Live you get character progression (in MW2 you actually level up and get new gear) and you are always in contact with your friends via voice chat that is built into every game...and if you are in a different game you can still chat...
Guild Wars Cities and outposts are divided amongst districts. From what I understand of the technology, each district is its own part of the cluster, and in order to move districts (to be with your friends for example), it requires its own loading screen as your character is loaded into the section.
Blizzard's Phasing is very Different however. I can't think of any MMO that's done that before. When things have the appearance of changing in other games, it is usually the result of a patch or is located in an instance.
And I haven't played XBox live games to know how the voice chat system works. good to know, but you're still using third party sofware (the XBL chat service) instead of one the developers created specifically for the game. Although I could be wrong if you are in fact able to communicate with someone from XBL on the PSN network through a game like MW2 (dunno if any games out there share game servers yet or not between the two).