I've only been back through old instances a few times, and every single one was just depressing. My guild, a few weeks ago, decided to run MC with about 30 people, just so some alts could get geared and we could spit in the face of mobs who'd given us trouble in the past. But I was glad I had to leave for some real life stuff halfway through-- we were taking down bosses without even blinking, and trash mobs dropped too fast for me to even get a cast off. We fought for weeks over this?
I've also done one run of UBRS (to get some people attuned to BWL, which I passed on after that bad MC experience), and one of Stratholme just the other night-- my 61 rogue, a 65 priest and a 70 hunter farming Runecloth and Righteous Orbs tore both sides of that place apart so quickly that I was bored the whole time (the Eye showed up, and we just killed the adds it brought). The one place I'd actually gain something from, Zul'Gurub (I've got quest pieces that I need rep to turn in yet), I haven't had the guts to go back to yet-- I liked running the instance so much when it was challenging that I feel like going back there now with ten or even five people and tearing it apart would be like visiting the house you grew up in: everything just seems so much smaller and more depressing.
Have you been back to the old content? Is there any challenging fun to be gotten from it at all? Originally, I was thinking it would be nice if Blizzard could reinvigorate these old instances, and I guess some kind of heroic mode would still be fun. But at this point, I'm almost of the opinion that they should just be put out of their misery.