Bornakk drops some serious knowledge about the difference between Achievements and Feats of Strength (both of which, apparently, are showing up as of patch 3.0). Achievements grant points, and are apparently things that you can accomplish in the game while they're given out -- if there is an achievement on the list, you can earn it. But Feats of Strength are different; not only do they not give points, but "some of them may no longer be possible to do." So Feats of Strength may include things like opening the AQ gates, and so on. The good news is that if there's an Achievement for something in the game, Blizzard will make sure that it's possible to jump through the hoops you need to do it.
But the bad news is that there's really no way to know whether you'll get credit for either Achievements or Feats of Strength until the system shows up on the live realms. As we've heard, Blizzard is going to try to be as retroactive as possible -- they'll be looking at quests and maybe even equipment to determine who's killed which boss in the past. But nothing, as Bornakk says, is guaranteed -- you may get one achievement for an old boss, but you may need to redo another old boss again.
There's one more great thing coming out of all of this as well -- hopefully, Achievements and Feats of Strength both will drive traffic back to the old content, and people who've never run Naxx or AQ, for example, will be able to find regular groups for "Achievement runs." Even people who've already downed the content will want to do back if their Feats of Strength don't register, so lots of people who didn't should have new chances to see it.