Alright I am not going to repeat much of what has been said by Elim and other ava members. I do agree with them but I think they managed to comment enough on those specific posts.
Tanks are normally warriors for the very reason that they are built in such a way as to be tanks normally. The entire class 'warrior' is a tanking class. They were built for that. enough said.
What can be said is that sometimes you will not have a warrior tank available for a certain boss at a certain time. Therefore, other people try to tank with other classes instead. One of these is the ranger class. The way most people build their ranger... they cant take one hit, let alone a volley. XD Others however CAN tank, especially with proper druid support. It is more difficult for classes other than warriors to keep agro for an extended period of time. I normally end up competing for agro in a bossing group between the 'tank' and other dps. Tanking is twofold. 1) keeping agro, and 2) being able to sustain damage over a period of time without dying. Most 'tanks' suck at number 1, others at number 2. The great tanks are the ones who manage to get both of these abilities within their builds and keep it that way.
Due to the low vit of the other classes, they normally suck at number 2. However number 1 can also be most difficult without many good damage or taunting skills. This is why it is so difficult for many to be tanks and why there are so few real tanks and almost no good tanks from classes other than the warrior class.