Ok, first off, this is the second post in a few minutes that I'm writing in response to reading a post from Tobold's. I cant believe this gem of a blog has been out there for this long without me reading it yet. Go check it out.
Back in January he did a post about how XP is handed out for killing mobs and how it really encourages risk averse (perhaps fun averse as well?) behavior. Sure you get a bit more XP for fighting higher level mobs, but not enough to make up for the difficulty or mana-recovery down time. Also, fighting two mobs at the same time is way harder than fighting one at a time (for most classes), but the XP gain is the same for both situations.
That got me to thinking....
When you're grouped up, you individually get less XP for the kill.
Have you ever been out in a area, either grinding or working on a "kill X mobs" quest, and you come across another player who is clearly doing the same thing? So you ask them if they want to group up and they answer "nah, xp is better solo".
If you want to encourage grouping and social situations (it is a massively online game and not a solitaire clone, afterall), why not offer a slight XP bonus if you are in a group of appropriately levelled characters?
I think the issue is that Blizz currently thinks they are giving a bonus to the group. If you kill the mob solo, you get X amount of xp. If you group up and kill it with two people, you both get Y amount of xp, and in general 2*Y > X, so Blizzard views the situation as a bonus. You individually got less XP but the universe of players as a whole got more XP due to your team work.
What would happen if they made it so that Y > X, even if only by a drop?
I think you'd see more collaboration and grouping while levelling, and in doing so, you'd have people levelling faster, which appears to be what they want at the Azeroth levels anyway.
Add to that a change that grants the drops from "gather Z drops off of X mobs" quests to all players in the group, and you'll have grouping become a sweet option for powerlevelling.
And hell, they could even do some special fun stuff, like you only get the bonus (or perhaps just a bigger bonus) if you're in a group with at least one of each party role (tank, healer, DPS). To make it fair on hybrid classes, they could build a little flag interface where the party leader can flag each player as to what role they're playing. Of course, this would be a little complex as it would have to make sure it doesn't allow a Rogue to be specified as the healer and so on. But I'm sure they could figure out a way to reward the group for trying to level up some tanks and healers, rather than just giving them better solo'ability (like +healing also translates to some +damage, which they did in a recent patch). The only reward of group play I can think of is in loot from dungeons, which is only accessible, theoretically, if you have the right blend.