You are always wanted, that is for sure! Dosent matter what content we are talking about but to make this easy we can focus on LFG, we can skip the part about what class you are and just focus on the role tank.
Im a tank since Karazhan on multiple classes and for me it has slowly become more and more fun but also very mentally demanding to have this role. Its not only that people are dependent on you but you are also the one that decides everything that is going to happen. For example, with the right dps, the right healer you can do faster pulls, you can do bigger pulls and in the end people will love you for it.
In the end, your gear and your pace is whats matter.
There is so many things that can be bad. Everything from slacky dps to people that ignore you completely and just rush in to pull everything that they can see or the once that cant manage there aggro at all. These are the main problems i have when running in LFG. Healers are a different story all together, you should watch out for the healers that is totally quite.. in general im going to say that they are the once you need to keep a eye on, well atleast there mana. A healer never says that its hard to keep you up or anyone else in the group.
Just to be sure, whisper the healer and ask him if he is having problems.
The ninjas, more and more are we seeing people that dont respect rolls or understand gear. Many just look on the itemlevel and ignore the stats, a item that has hit and mastery isnt always a dps item and works just as good for a tank. With the current gear we do gear more like a dps and some dps might not accept that.
Be ready to explain why you needed on a item that might look like it was designed for a dps.