Druids are pretty fun. The feral tree is the generally accepted leveling tree. Actually, on my server Feral seems to be the generally accepted tree ...period, but that's a different story altogether. Unlike Prot for a Warrior, the Feral tree allows you to both tank and melee DPS, so you'll have quite a bit of instance utility as well as soloing options. As far as builds are concerned, here's what I did:
level 21
level 30
At that point you can heal, tank, and do melee dps. From there you can drop points in either Feral or Resto depending on what you enjoy more. I elected to do a pretty even split because I was instancing a lot and it was helpfull to be able to tank or heal depending on the group makeup.
If anything, I've found Druids to be even easier to level than Warlocks. Pull with a spell and pop a HoT on yourself and then switch to Bear or Cat depending on how many mobs you just pulled. Wash, rinse, repeat.
As far as a good partner class is concerned, I liked leveling my Druid with my wife's Shaman. That's mainly because we were both able to DPS and heal if need be. It made for a very flexible pairing.