Herald of the Titans earnt before the release of Cataclysm. Had someone ask me when I got it recently, and I said WOTLK and he told me the achievement wasn't possible to get back then, cried a little bit inside. However I earnt it after the Cataclysm system patch with the introduction of Holy Power and the like so I don't consider it a proper one, but at least I earnt it back then without leveling an alt for it.
A Tribute to Insanity (10man) - earnt in early ICC days so again not proper but over 75% of the people I did this with have since quit and being on an underpopulated server very few people got this ever.
Champion of Ulduar - basically The Undying but in Ulduar. Got all the bosses for this in Ulduar days except Mimiron for which I had to wait til early ICC again, because we kept running Ulduar for it and different people each week purposely ran into Mimiron's one shot abilities thinking it was funny. Great guild I was in, in terms of attitudes and people. Don't think more than 4 or 5 guilds got this on the server and all the others except mine were primarily 25man guilds and we weren't, so they got the 25man one too (and then almost all of them promptly quit, I have NEVER seen anyone wear Champ of Uld on my server except me and 1 person wearing Conqueror).