The majority of the time your standard EHP set will suffice, I even use my EHP set on trash. However, some people have fairly intense threat setups with lots of armor pen and crap that they use for trash. I frankly don't care about trash, so I've never put forth the effort.
There's also avoidance sets, there are a very small number (in my opinion) of fights that actually do favor avoidance, so far the only 1 in ICC so far is Gunship if you're single tanking the enemy ship, if you're doing I'm on a Boat you want to stack stam out the butt.
There are also two different classes of EHP, physical EHP (or PEH) and magical EHP (or MEH). Basically if a fight does a lot of magic damage and not a lot of physical damage, then the value of armor goes down, and the value of stam and magic resistance goes up.
Tanks also will carry different sets for anub'arak, specifically heroic 25 man, where the MT will go with a lot of resistance enchants and favor armor way over stamina because stam makes leeching swarm hit harder, so it becomes non-optimal, and the OT will sack stam at all costs and stack avoidance and block and block value for the adds.
There are also just standard threat offsets that don't really deal with armor pen or anything much, they just focus on getting hit cap and expertise cap. It will typically end up with decreased EHP, but if it is on a fight where your EHP really doesn't matter (like Lord Jaraxxus) then you might as well throw on tank gear that allows you to get to the caps.
Some people might even use a "taunt" set, where they put on hit gear to make sure that taunt doesn't miss. There's a footnote in my guide that breaks down the levels of hit you would need to guarantee taunts landing provided taunt glyph and various raid buffs.
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