Feral druids make excellent tanks. They can actually get much higher armor than warriors if they are geared properly. Even with extremely good tanks, many endgame enemies can kill them in about two good hits. Sometimes priests just can't keep up with the heals. That's why I developed a team tanking system with my guild. It was extremely effective and we were able to do very well in Karazhan with comparatively low level gear.
The key is to have both a druid and warrior tanking at the same time. Both of you should do damage but keep the warrior's aggro levels above those of the druid. When the warrior is low on health, the druid uses growl. Growl forces the enemy to attack the druid for a few seconds regardless of its aggro. This effect soon fades and it will go back to beating on the warrior. My warrior and I would bounce aggro back and forth so quickly that each of us only took a couple of hits. We each had a team of healers who could easily keep up with the heals since there was time to heal us. We would keep this pattern up and did very well against bosses that should have torn us to shreds.