The real irony is, I prefer prot spec and would love to invest heavily in the tree, but my horde warrior is much, much more effective at both PvP and soloing/questing as an arms/prot hybrid (15 points in protection) than he would be specced purely for tanking. I already have one character who is set up purely for instancing (my 70 resto shaman) and frankly, I need someone who can generate the cash we need to stay afloat for Kara runs. And so, I tanked Shattered Halls as an Arms warrior and when we were done and Bladefist was dead, I went on my merry way, the mercenary warrior who tanks random PuG's for blue drops and who never gets that damn fist weapon. I wanted to offer this advice for the legions of people who seem unable to find a tank.
If you want a tank, then don't give the tank crap about his spec once he joins the party. Don't tell her how to tank the instance. Don't offer up tips based on other tanks you've seen or constantly complain that the run isn't going as fast as it did that time you had the Feral tank it. Because we don't care. I am not in your PuG because I care one iota about you as a person. When I do guild runs or runs with my friends, I might care about how much fun they're having, but with a stranger I am there for as much money and loot as I can get and as few wipes as I can arrange. If I'm not specced to your liking, I don't care. If we're not going fast enough for you, I don't care. I am sitting here at 493 defense beating on mobs because I have something to get out of it in most cases, same as the rogue daggering it up or bludgeoning it, same as the warlock DoTting it up, same as the shaman in the back casting Earth Shield on me to help soak up the damage.
This is pretty general advice, I realize, and it applies to any tank. If you're curious about the paladin who is going to be tanking the instance, ask her up front, don't wait until mobs are being pulled. Don't wait until the druid casts thorns and switches form to get ready for some faerie fire to try and argue that the warrior with the 2h weapon should pull out his shield - what do you think the druid decided to go all furry for? It wasn't so you could see his excellently large bear behind up close and personal. Yes, I'm sure the texture is very well rendered, but he's actually intending to hit things like that.
Yes, I'm aware that often, it's the tank being a jackass. I've got a healer, I know there are tanks out there who will say things like 'Shield Slam? What's that?' or who don't know about the Shield Block/Revenge combination, who won't wait for mana, can't find Sunder Armor with both his hands and Google maps, who don't bring consumables, and sometimes who are just generally terrible because they're new to a tanking class and don't know what they're doing. Sometimes these people can learn if you bother to try and help them, and sometimes they can't. It's certainly not the role of a PuG to become a shop in tanking basics. But just as I don't talk to every rogue I play with as if he or she is brain dead because of the many, many times a rogue has wiped a group by going on to engage another mob while we were still recovering from the last fight... just as I don't treat all mages, or all of any class or role as if they were mindless idiots because of the multitude of mindless idiots I've experienced, I'm afraid I'm going to have to ask that you wait to be jerks to me until we've done a few pulls first.
Other things to keep in mind: this is a personal quirk, but if you're telling racist jokes, being a homophobe, whatever it is that personally bothers or offends me, you'll get one warning from me and if you keep it up I'll drop group. See, while I don't personally think there is a tank shortage, I'm not silly enough to let the power inherent in the common perception of tanks being hard to find go to waste. I humbly encourage every tank, be he warrior, druid or paladin to make use of this situation. If they're asking you to tank, don't let them disrespect you. In return, if you're getting treated fairly, do your best for them and treat them fairly as well.
Part of the reason why it seems hard to find a tank for a PuG is because, after being burned enough times by them, tanks tend to stick to guild groups. I may tank less if I do that, but when I do tank, I know it will be appreciated and my work will be respected. Why would I trade that for being dumped on before I even step into the instance? The tank's time is as valuable to him as yours is to you. In many cases, if you treat a tank well, much like a wild bear you coax forth from the wilderness with honey and other treats, you'll quickly find yourself with a steadfast pal who will endure all manner of crappy runs with you while trading whispers about how incredibly awful that other guy is.