I'm at something of a crossroads at the moment. My gear is getting better and better (I now function more or less as the main tank for my guild.) However, I do like to hang out at theorycrafting sites. In general a tank will equip the best items they have resulting in the biggest HP, certain (required) levels of defense, and generally very high dodge and parry.
In practise high-end tanks have several sets of gear they carry around, or at least some switch-out items for specific slots (rings, trinkets usually.) As I said my gear is getting better and now I'm at the point where I might see benefits from constructing seperate sets of gear for different situations.
It's not particularly easy, I must say. Gem and enchant choices for one set may conflict with gem and enchant choices for other sets. For example, I may have a purple gem in a certain item to activate my meta gem, then get a problem if I switch that item out as the replacement may have a yellow gem, for example. Enchants are a problem too, esepcially chest and cloak enchants. For chestpieces the three main enchant choices for a tank are 22 defense, 275 health or +10 all stats. Any one of those may cause a problem, in one set I may have high defense, so actually want health or stats, whereas in another I may actually need the defense to get to the minimum of 540.
Even just maintaining one general purpose set is not so easy. When I get new items they may be upgrades but may require switching several other items to maintain defense minimum, Meta activation, hit rating etc.
In effect I'm turning the problem of getting the gear into a logistical problem of turning the various items into a set of gear. It's not easy being a tank ;)