My guild has a mythic dungeon night once a week. Not much different than heroic if it's the regulars, but some weeks I'm carrying guildies who haven't done them and that can be challenging all over again, especially if I'm tanking on an alt instead of my main. Not challenging like banging head on wall, but challenging as in my health bar dips, have to use CDs, and occasionally have to rez people after fights.
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The other thing with heroics is that daily timer. Makes me feel like there's 100 ring valor I'll lose if I don't take 5-10 minutes per alt and knock them out each day. If not for that, I'd be as likely to run heroics as I am to do Dalaran cooking dailies.