Brief infodump: Lux and I have pretty much been playing games together for a long time. WoW is no exception. We've done most major things together, and our guild is used to achievements popping in twos (and I have to admit, I found it hilarious when we got Two Humps). We even got our level 80 achievement on the same kill in HoS, became Explorers in a bike built for two, and, of course, got just about every raid achievement we've ever earned--together.
So, the new questlines from 80-85 are mildly a pain from a duo-perspective. With heavy phasing and excessive breadcrumbs, we had to be in constant communication. More than once, one of us would take a quest and the quest-giver would disappear or we'd get thrown into a plane and land half-way around the zone. Do you have this quest? Did you get credit for that? You're not on my map anymore, where did you go?
It's not unplayable, by any means, but I think it would be difficult for two players who just wanted to quest together for a session or two to do so. If they weren't on the EXACT SAME LEG of the zone, they wouldn't be able to see each other, let alone get the same quests. Their best bet would be a new zone altogether, or to avoid questing and just queue (oh but which dungeon- do you have this one discovered?)
Lux and I were already used to quest monogamy so it wasn't a big deal for us, though it's annoying to take a quest and have him disappear because I reached a different phase.
Epic... all by myself
For fun, we made some goblins to see the new starting areas. I made (yet another) priest and he made (another) warrior. I was not prepared for how difficult it would be to force lowbie leveling into a team experience.
First, the phasing. I don't think I would have noticed it at all had it not been how many times I looked over at his screen and saw something entirely different. If I were by myself, the phasing would have been seamless and the storyline was really neat. But the starting area for goblins is more like a single player game than an MMO. As such, we spent a lot of time basically doing the same thing in the same place but not at all doing the same thing or in the same place.
Second, the weird way we got credit for things. Some quests functioned like normal, where we only had to slay 10 rats. But there were a lot of quests that, in the rest of Azeroth, we would normally be able to get credit for things together and it just didn't happen that way. We each had to cajole our own party-goers, run down looters, break into the bank, and yes, we did have to blow up the mansion twice in succession (though we each only saw it once).
Some of those I can understand, but some just seemed buggy as all get-out. Really? I need to pull the lever, too? We need to free double the captives? We need to kill this guy again to prove he's dead?
(Granted, it makes more sense that a quest mob would only drop his head once. But then if were were following logic here, why couldn't we just bring his head back together?)
And to make matters worse, it wasn't all the quests that worked that "didn't work", some did work they way they normally do. The ones that didn't were just often enough to keep us in a constant state of confusion. Did you get that? Yeah, I got the combobulator. No, did you get credit for the Goblin Zombie? No... Wait, yeah I did. Er, no. I still need 5. Is that what you need?
Wait, huh? What combobulator?
Our lowbies are level 21 now and even into Azshara we were still dealing with the confusion. (As an aside, I really do think that Blizzard did a fantastic job with new leveling. Both my shaman and priest got abilities in a timely manner, I enjoy the popup at level that tells me what I can train next. Getting a core ability at 10 is really neat, and the talent trees are more fun to level in).
But really, the new questing paradigm at low levels has been more like each of us playing our own game in the same room than us playing together. Really, Blizzard, you want us to escort the same caravan... again? Not fun.