Multiple Gearsets

so one thing I should probably mention is that my main all through TBC was my mage.  My warrior was a product of getting bored with my guild's slow progression (we aren't a raiding guild, more of a social guild that raids), but now my Warrior is more or less my main.

Why should you have multiple gearsets?

so I'm pretty sure any serious warrior tank would laugh at this question, and even before I read what Vene posted about it, I would never have asked that question.  I know warriors need multiple gearsets because tanking is all about being the best you can for the encounter you're facing.  Unlike DPS, who can for the most part use the same gear and rotation for any boss encounter, tanks have to think about things like "How hard does this boss hit?" "Do I have to move a lot?" "Is there an excess of physical or magic damage?" and while simply knowing the answers to these questions will help you adjust your rotation, or perhaps help your healers know how/when to heal you to get the most out of it, as a tank you can take it one step further and set up your gear to match the encounter.

How many gear sets do I need?

This is a question that's better left up to you, but a good rule of thumb is, don't ever throw away tanking armor.  I'm not talking about that lvl 52 green chestpiece you have sitting in your bag.  Obviously you shouldn't ever need to make use of that.  But to use a current example, the crafted blue gear at lvl 80 (or lvl 78) has extreme amounts of defense on it.  Some pieces have upwards of 75 defense on one piece of gear.  While it's lacking in other stats, it's still worth hanging onto.  Let's say you're putting together your threat set, and you find yourself short on defense, well, sometimes it's worth it to put on that blue piece to round out your gear, rather than using an epic that means you have to trade out 3 or 4 other great pieces just to hit 540 def.

Personally I have 8 gearsets right now, and 6 of them are for tanking.  I have a generally balanced best in slot set, a block set, a boss threat set, an avoidance set, an EH set, and a Stam set.  I plan to put together a raid trash set and, if I can, heroic versions of some of those sets, so that I can use better pieces without having to worry quite so much about defense.