Now we've got a more complete picture of which achievements will be extended with new tiers and which will languish as they are. Here's a quick rundown:
Exalted Reputations: A new tier has been added to 45 reputations
Honorable Kills: A new tier has been added for 250k honorable kills
Battleground Wins: Still have an achievement for 1 and an achievement for 100 with nothing in between and no new tier added on
Got My Mind on My Money: Two new tiers added for 50k and 100k gold looted
Cooking Recipes: New tier for 200 recipes learned
Companion Pets: No new tier, still maxes out at 75 pets collected
Mounts: No new tier, still maxes out at 100 mounts collected
Tabards: No new tier, still maxes out at 25 tabards
Quests: No new tiers, still 3,000 quests completed and 1,000 daily quests completed
Fish: Still just 1000 fish caught to max out
Of course there is a feat for level 85 and 525 skill in various skills. We knew those were coming.
This list strikes me as very haphazard and random. Three different things were done with different multi-tier achievements. Some were left alone entirely, Some were given a new tier that a serious collector can easily have achieved by the time 4.0 hits, and some were given one or more new tiers that feature really outrageous numbers.
It's really disappointing to not have a new mount or pet number to shoot for. 75 is actually a really small number of pets, and 100 will become fairly easy in Cataclysm with the new mounts that are added (for Horde anyway). New new tier of quests or daily quests is quite baffling to me. Presumably they will be adding a very large number of quests to the game, why not ratchet up the high-water mark? I'm glad to see I'll be getting 10 more points for the number of exalted reputations I already have, but compared to what they did with some of the others, it doesn't pose a great new challenge.
Now gold looted and honorable kills are where they got it really, really right. 100k gold is a huge number, and while I don't doubt there are people who have looted that much already, there aren't a lot of them. Similarly, 250k honorable kills is a very big step up from 100k indeed. Just for fun I went and looked at the first player to get the Battlemaster achievement on the armory. He has only about 150k honorable kills. The player that appears to be the world first to get 100k honorable kills1 only had 155k now. 250k is really something to shoot for. I'd try to do similar benchmarking on gold looted but it doesn't show that statistic on the armory. All I know is that I have around 56k gold looted and I went to some pretty extreme lengths to get there.
Of course honorable kills and gold looted are both unlimited. You can always get more. Mounts, exalted reputations, pets and quests completed have caps. For mounts and pets the caps are noticeably lower for some than for others, since many mounts and pets have been phased out of the game. Still, anyone who was able to get 100 mounts in wrath should be able to pull of 120 in Cataclysm. Anyone who got 75 pets in Wrath should be able to get 100 in Cataclysm (100 is pretty easy right now). And leaving daily quests at 1000 seems really low. That number could easily go to 2000 or 3000, if not more. In reality, 10,000 would be more comparable to the 100k gold or 250k honorable kills achievements. It would require only just over a year of maxing out daily quests to get it done from scratch. If you can get 250k honorable kills or 100k gold looted in a year of diligent play I'll be impressed.
New tiers of existing achievements are probably really, really easy to add to the game, so I guess I won't be surprised if there are a couple more in the works. I really hope there are, because right now I don't feel like Cataclysm is bringing with it that much to achieve. New level cap, new skill, new raids, and all excluded, of course.
1. I assume this means he was the first player with 100k kills to log in after the achievement system went live, but still, 100k kills before the achievement system went live and being obviously still active in the game should put you pretty high up in the race.