Take Ahn'kahet: The Old Kingdom. Prior to the random heroic dungeons, I probably ran this place a grand total of five times. My warrior tank got the bracers on his fist run through it, and I did it on a few more alts just to get the Northrend Dungeon Hero achievement. Other than that, when it was the heroic daily I'd just skip it and find something else to do. Nothing annoyed me more than the Insanity that Hearld Volazj casts, and the trash pulls got on my nerves when the DPS couldn't wait for the mobs to come to the tank, inevitably leading to a wipe.
In short, it was my least favorite instance and I was able to just ignore it. From the best I can tell, before these past two weeks, the last time I set foot in it was in May 2009.
But now, the random dungeon finder has forced me to relearn the place and deal with all its mechanics. I've found ways to deal with the Hearld Volazj's abilities on my paladin tank, and I've gotten rather vocal about the group staying put as I pull trash packs back to them. And just as important as knowing the mechanics is leading the run as a tank. For instance, if I see a DPS move up with me, I'll stop and ask him to go back. If he doesn't listen, I'll ask again politely. Third time around with a random DPS not listening I'll just threaten to kick, and by that point the group is pretty annoyed with him anyways. The next thing you know, he's out the door with a successful vote to kick. The next DPS that comes in always seems to behave.
So what dungeons have you had to relearn dear readers? And better yet, what ones have you had to change your tactics on with the introduction of the random dungeon finder?