I love this, as I seem to love most of the things that Mania does. She's just posted that she's put together a whole guild of hunters, all with XP turned off at different levels, for one purpose: testing pets as they move up the leveling ranks. She says she cheated a little bit to do it -- transferred alts from other realms, and it's Alliance-only -- but just having the idea to put something like this together is super creative. We've talked about people who've leveled up one of every class before, but I never considered just how much the XP-off option changes the game in terms of what Mania calls "special purpose characters."
Of course, raiding at level 60 is one way to use it, but you can go even lower than that -- want to farm Runecloth without it getting too boring? Roll up a death knight, and leave him in Felwood to grind on Furbolgs. Really love running, really running, Scarlet Monastery with your friends? You can all roll characters to 39 (or lower, if you're looking for a regular challenge), turn XP off, and leave them camped outside the instance. Turning XP off means you can create characters for almost any purpose, and having heirloom items (especially if you buy cloth, which any alt can wear, even if it means they take an armor hit in some cases) means that leveling them up doesn't take more than a few days of free time. Lots of interesting ideas to play around with there for sure.