Get X pets. 10 points
Now Y pets. 20 points (but you only get 10 more since it's going to 20 from 10).
And so on.
And throughout the expansions they are going to keep adding more achievements like this. 200 pets will come out sometime, and 250, and so forth.
I'm just saying that some achievements need to hit a wall eventually. A great example is 250k HKs. That's insane in the first place. Anyone who has that many HKs is someone whose played their toon since vanilla, is a hardcore PvPer in a serious PvP only guild, has multiple people playing their account, and/or just has way too much free time to get there. I highly doubt many, if any, toons made during mid-late TBC are close to it.
Achievements are fun to farm up and do. It's nice to hit that final wall of achievements and say I hit it, and some achievements should later be given further progression. Some shouldn't.
I'd really cry if I ever saw something like 500,000 HKs. I only know of one player on my server who is close to that, and guess what? Like 4 people play him, and he held Grand Marshal for 6 straight months in vanilla.
edit: Oh, and he had 100 victories in TB before February.