I spent enough time in MC, BWL and AQ40 before TBC to know that the only time I truly want to play in those zones again is for the "LOL Hey guys let's go see how far we can get into X". It's fun for a night to see what we can do - and honestly the less people there, the better. It's more fun to see how few you need these days.
With the exception of C'thun and the Twin Emps, most of AQ40 can just disappear into oblivion.
I normally take the time to attune myself for the old instances for the lawlruns, but I would quit before I would forcefully run them on each of my alts so I can move on to the newest thing.
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Naxx has always been awesome. I would never be opposed to running it seriously ever - on any character. Even if I always failed at the Heigan dance.

((I normally got the lethal final port into the hallway before a dance phase.))
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