As JeeBar pointed out, the number one rule is to reach uncrittable status which is 415 def. Resilience contributes to this as well so if you have any pvp gear, you actually become uncrittable well before 415 def. I suggest reading up about defense and resilience on wowwiki or something. (Sorry I'm lazy or I would offer you a link or two to check out)
Assuming you are uncrittable, there are several camps of thought on how to gem/enchant:
1. All Stamina - Your health pool is huge, but you fall a bit short on avoidance and threat generation (via crits). Thus probably the least popular method.
2. All Agility - You're a dodgin crit machine. A fairly popular strategy because agility is just that awesome, but a string of crushing blows from a hard hitting boss (it happens even to the best of us) will erase your health pool and run your healer(s) oom. This is how I initially worked it, but am now working towards:
3. Agility / Stam - Shifting Nightseyes are your friends. Some dodge, some crit, some hit points... A nice rounded build that I think will serve you well.
Dont let any trunk monkeys convince you to add strength or +hit or dodge rating... Take it from a long-time rawr-tank: all you need is the real thing, straight from the source: agil and stam. Many respectable bears even say to ignore socket bonuses and to gem everything with shifting nightseyes. I like my socket bonuses... but... they may have a point. In the end, socket bonuses don't really make that much of a differene to fret over.
My .02
-B