What class? And how much DPS gear? Three or fewer pieces won't be bad at all if they have a good item level, but if you've got over half DPS gear you may want to either buy the 450 crafted gear or run scenarios in tank spec and DPS gear for the random pieces you get from those.
I know very little about warriors. DKs, you'll be fine if the DPS gear has mastery; reforge the other stat to avoidance. In 5.0 paladins could actually tank effectively in haste DPS gear, although you wanted to go full haste for that; the increase in HP generation (and usage of SotR) makes up completely for the loss of avoidance. (Double check to make sure this hasn't been nerfed, before you try this. My paladin is holy/ret!)
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If you have too many and you find yourself having me heal you - if its bad I might say something, but probably just cringe and heal through it. Honestly - if you know what you're doing as a tank, and understand how to rotate CDs or use them in the right situations and have a decent healer it'll probably be fine.
That said, I wouldn't recommend doing it if you're more than half dps gear