I had made short mention of this once before, but as time has gone on I think it's worth a full-fledged post of its own.
"Am I ready to tank ________?" This is a question that pops up quite frequently on Maintankadin. People with varying degrees of progression, with various levels of crit immunity, crush immunity, hit points, avoidance, spell damage, and whatever else happens to be important to them want to know if they're ready to tank the next boss. They want to know if they meet the "minimum requirements." Try as I may, I have a hard time looking at questions like this as anything other than laziness. And that's fine; normally I wouldn't mind at all if you'd rather just get a quick and easy answer and move on with your life. I'd make a sticky that says "You need this much stuff to do things" and point people to that. That's what I'm here for.
Unfortunately for me and everyone else, however, there are no quick and easy answers to that question. Worse, the only person qualified to answer that question is the person who asked it. There's just too many variables to the equation. Even if I just said something like "You need 12k HP to tank Prince", I'd get ten people saying "No way, that's not nearly enough" and ten people saying "I tanked him with way less than that." Your gear is only one part of a much greater picture; an important part, but a singular one nonetheless.
The fact is there's more going on than just you standing in front of something nasty letting him (or her) whap you in the face. Your healers wear gear too, and the necessary quality of your gear depends largely on the quality of their gear. Same goes for your DPS to a certain degree - you need to be able to hold aggro or you'll just be holding them back, and healers going OOM because the fight took too long is just as bad as healers going OOM because you took too much damage.
On top of all of that, there's also player skill to consider. Maybe your healers are bad. Maybe your DPS is bad. Maybe you're bad. I'm not going to hold that against anyone, but generally speaking, at least in PVE, you can take a bad situation, throw some gear at it, and come out on top.
Honestly, I think the whole thing comes from a poor perspective on tanking. Until you're wearing the best gear available to you, with the best enchants and best gems, there are upgrades. You should make it your goal to get those upgrades. Don't worry about meeting any artificial "minimums," just focus on getting the best gear you can. Unless you're a freshly ebayed level 70 with nothing but some random greens that the leveling service happened to equip (in which case, you don't deserve to be tanking things), someone else has probably already tanked whatever it is you're looking at, and in worse gear. Whether it's a good idea or not depends on the 4, or 9, or 24 other people you're there with. Leave that decision up to the raid leader, or if you are the raid leader, you should have a handle on how things have been going - make a judgement call. That's your job.