Being able to fill any role in a group.
Seriously, I'm only level 28 right now, but I've been in maybe 7-8 instance groups already. Whenever I have time for an instance, I put up the LFG or check to see who is LFM. I have a healing gear set, and a DPS/tanking set(I play PvP, so I like to have all the stamina I can). I also never healed before playing druid, and it seems like I'm always wanted. Although I never thought it was possible, healing is awesome fun!
The only problem seems to be with drops. I can use both upgrades to my healing/intellect set, and my tanking/DPS set, so I am really careful about which items I need on. Just the other day, I went into SFK and got 3 blue drops, two healing, and one DPS.(I was healing, so I let the mage/lock know I would need roll against him, and the rogue/hunter both had the DPS drop I got).
I just felt like I was taking everything. I was happy about my rolls, but unhappy that others didn't get their just desserts.