can buff and heal, can be a cat for damage output or a bear for more tanking. has CC so can root an enemy for a few seconds. and you use different armor types so in pve should be easy who gets what drops. druid can also pull with its spells.
Pretty much all the duos I did with my Warrior worked well in PvP. Warrior and Rogue seemed to work really well, Mortal Striking a stunned enemy is fun. Warrior and Shaman combo can also be devastating. My brother played a Priest though so from my experience in our duos that worked the best. We could take on 3 level 60s quite easily, sometimes 4 if they were an unbalanced group or didn't know what they were doing. Anything is possible with recklessness, kind of a cheat ability really.
We took on a fg of 52-55s once too which was quite fun.
If you want to be best in most PvP situations, I'd say go for Warrior+Priest. But you say you want to do damage too, so I'd go for Warrior+Shaman.