It's bothered me that blizzard rewards people for grouping up in all areas of WoW except one, leveling. I've done two expansions now boxing and solo leveling thru all of WoW. Solo I can quest level in about 2/3rd the time it takes to do a team.

One: kill exp is substantially higher. Yes a group kills faster, but mob density limits kill speed. A group just has to do a lot more running to even compete.
Two: collection quests fucking suck as a group. Sometimes they will let everyone loot the item, but typically it's one loot per kill.

Now as I see Cataclysm coming closer I'm excited to level my first group, but honestly I don't look forward to the groups after that. I'm not in beta now, so these issues could be fixed already, but I doubt it.
To prevent exploitation they could implement a timer that reduces xp gain based on your kill rate. The other issue I just don't see much exploitation occurring.
The positives of this for non boxers is that friends can level together and not feel like they are dragging each other down. Thoughts?