Originally Posted by
Mercurio
I haven't tried any myself, but the vast majority should be boxable. Typically, the content in a previous expansion is pretty trivial. You may have to have do minor things like have your pets tank for Immerseus, divide your group into two for Spoils, etc. but you should be fine. There may be some odd mechanics like the last boss in Terrace of the Endless Spring when he teleports your guys around, but even with that you will probably down most bosses before they can start to do their interesting abilities.
As an example, during MoP, I took my guys into Dragon Soul (hardest raid of the previous expansion) and had no trouble at all cranking out all the heroic achievements. And that was before the stat squish which made things significantly easier. There is definitely no challenge for a boxer in the old raids, but they are really good for an afternoon of relaxing fun to see all that cool content.