Crowding is the only issue I see. It's harder to find a good spot for a group of six. I've been playing a druid with a mage partner and we both gain faster exp together than we do solo (although the edge probably goes to me... might be bigger but I tend to push the envelope more). We almost don't stop when grouped. Her pet and my buffs allow us to pretty much keep going non-stop... IF there are available pulls. I'm familiar enough with the old world zones that we've done well enough so far but I see that changing as we level up and run into more contention.
Fortunately for me, I'm only leveling the druid so I can have a runner. I'll probably switch to a paladin (yeah, Im sick in the head that way). At that point, my grouping priorities change a lot. I have enough exp to solo a pally effectively but I like dungeons and grouping in dungeons and the loot in dungeons. Of course, you don't play a hybrid if leveling is your chief aim.
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