WoW Provider has a nice list of all the current achievements as well as a all the Talent trees for each previous patch. WoWHead also has a nice big list of all achievements. Notice that the factions are shown double however.
For example there's the Classic Dungeonmaster achievement which requires you to complete every dungeon from level 10-60. Deadmines - Stratholme. And the Classic Raider achievement which requires you to complete Onyxia, Molten Core, ZG, TAQ and more. Naxx is not listed, as it will be moved to Northrend.
Most such achievements can be awarded by the game by looking at your quest history. It knows which quests you completed. Such as the quest to kill the prince in Dire Maul, or Scarlet Monastery. Not all bosses have a quest to kill them however. Onyxia, Nefarian and C'thun for example are involved in some quests but these are not always available to all players. The bosses do drop an item which can be turned in for a reward and this quest will allow blizzard to track who has completed the achievement. Since the item is a single drop however, they are also adding the entire loot table of those last dungeon raid bosses. If you recently cleaned up your bank though.... you'll miss out.
Ok, for everything 25-person and above, we added the full loot table checks on BoP items - Gruul, Mag, Vashj, Kael, Illidan, Archimonde, Kil'Jaeden, Onyxia, Ragnaros, Nefarian, C'Thun
Dungeon bosses and anything that requires less than 25 people will check any quests but no items. We can not check the Heroic Daily Quests or Dungeon Daily Quests as they are repeatable and not tracked accurately by our system. (Source)
Also : You will gain some rewards but the points are not spendable.
The good guys over at the EJ forums have put together a list of quests you could look at for all pre-TBC instances, as well as all the TBC instances. Having completed those quests, should in theory award you the achievement related to each of those instances. The items you'll have to gather yourself however.
Also see: this post at MMOC.
Basically if you've managed to do this quest, you're mostly set on all the pre-TBC raids up to silithus. It starts with this quest. Lots of farming, collect a head from BWL, more farming and a bunch of expensive materials. It's probably easier to just go back and actually _do_ those dungeons when you hit level 80. Onyxia should be soloable.
TBH, blizzard should add an "ancient fruitcake" achievement. I still got several stacks of Graccu's Mince Meat Fruitcake from the very first wow christmas event. Surely that thing is hard as concrete by now. Even my hunter's boar doesn't want to eat it anymore, and that critter eats anything.