There aren't very many quests that you have to have somebody else with you to complete anymore.
This is the sort of philosophy we're going for with phasing, as we do feel it's an important tool for storytelling. We do recognize issues where we've "over-phased" zones in the past though, and we generally tried to take a more minimal approach to phasing in Cataclysm. While there is still a good deal of phasing involved in many of the zones, we tried to make each phase a little more intricate and less intrusive on group gameplay. We want to make sure that, in times when a group is required, phasing doesn't hold you back from grouping with friends.
I said a variant of "phase" in every sentence. You're welcome. :p
*cough* phase *cough*
I'm not sure this is working as well as you guys think. There are leveling quests that sometimes must be completed twice when in a group of two. An example is the tentacle "boss" in the, um "sarlacc pit event" in Twilight Highlands; I apologize that I cannot remember the quest name or even the boss that we fought. This happens a few places, but here's the one I actually remember well:
1. Each member of the group --two of us-- was in his own personal phase fighting his own copy, or instance, of the final objective: some boss tentacle.
2. When one member completes the event, the both members
are yanked onto the back of an earthen ring gryphon and flown far away from the objective; quest object still incomplete for the slowpokes.
3. Objective incomplete, the slower members must fly back down into the pit and fight the target from full health.
That use of instancing/phasing was extremely intrusive. As are the phase-borders occuring at different times for different group members that are in the same phase. "Follow me" doesn't work when my unit in the game world can be erased from a friends' client unexpectedly.
EDIT: Forgot to agree with other posters that in many places the phasing is smart, brilliant, transparent (ish) and well done. It's a good idea that is possibly being over-used still, and wasn't graced with an abundance of thorough QA before release.
Good examples, great examples, of it working perfectly also exist. Phasing Dragonmaw Port into a larger city while I'm "sent" back to report success in Orgrimmar? Brilliant, nearly transparent to the user, and excellent use of the technology.