Personally speaking, my burst and initial threat is ridiculous, and on bosses that require more than one tank (bloodboil) I typically start it off.
The only difference really being is I don't have as many "oh **** buttons" that a warrior has, as mentioned previously, nor can I take the high burst damage as well as a druid. But all 3 tanks are extremely good to have.
Our guild uses 2 well geared warriors, me as prot pally, and a feral druid. If someone misses a raid, its no big deal, as all 4 of us can tank kalecgos pretty equally. The druid seems to shine on that fight, but all of us seem to average out about the same. I can take the big hits, just not as well, and if I need a heal or something bad is about to happen, I'm usually screwed where a warrior can pull out fine.
I would never want just one type of tank, I think it's a huge factor having all 3 types in a raid. Especially since on certain fights where the paladin or druid aren't needed, they can do other things better than the warrior could if he wasn't tanking.
Did I mention I love my threat? Just wish I could take the same amount of damage as easily, but there are pro's and cons to each tank. I might be squishier than the other tank types, but I won't be getting pulled off of by the dps.