For fun I've been considering making one of my mages spec a... tank spec. Obviously not for raids but for HCs and general messing around. Not my fault other people can't tank HCs. It's possible to reach the 540 defence cap, although A LOT of gems are required as well as enchants. Using Tanking Neck/Back/Sword/Trinkets/Rings for a huge boost to defence/stamina. Could mess around with it more, especially with trinkets and certain amour to get another socket out of it in exchange for losing other stats. Used WoWHead's profiler to see what it's like as Rawr doesn't like me trying to make a mage tank lol. Got up to 30k health unbuffed. The main problem I see is cloth, god damn lack of armour. Got that up to about 6.5k unbuffed which would have a huge increase due to various talents (http://www.wowhead.com/talent#ocfbbdrZZhIfIsbzcrdx0st:jsnMVz is the talent build I came up with). 1500 or so from ice armour, I imagine another 2000 or so from the arcane fortitude although could be wrong. That's 10k armour, which isn't too bad. Seeing as with mana shield and ice barrier most attacks will be blocked anyway, if you can still get enough threat could probably tank most HCs using this spec with the right gear. Not going to do it though... no way in hell am I going to collect PvP gear.