@Mar: That's good, man. Congratulations. This kind of made me realize that rating these achievements might be a little more ... [more]
Indeed. But I think what Giant Bomb have done is pretty much as good as you can do. Sure, some achievements will be skewed based on the fact that there's some obscure games out there and they may have a lot less players than others. But for the most part it works pretty well.
Hopefully over time this will balance out as well. If this feature on Giant Bomb gets used to its potential, you'll find people trying to get the rarer rated obscure ones.
The majority of rare achievements seem to be rare because they're from games most people haven't played. Perhaps Giantbomb should ... [more]
This is certainly true for most. However the one I pasted above is most definitely the most difficult achievement I've unlocked on the 360. There are only about 20 or so people in the world with it too, and half of those are cheaters who have had their accounts banned.