One thing I enjoy doing is looking at the frequency of achievements. Oftentimes you can spot where the game got hard or unpleasant because there is a sudden drop off in the number of people with the next achievement.  It's by no means a perfect indicator, some game just don't dole them out evenly or there is a non-story achievement that is much harder than the others.  The biggest gap I've ever seen is in Braid.  With its stupid speed run achievement there is a 61.31% gap.  Avatar is probably the smallest with all 5 achievements only covering 1.59%.
 
Does anybody else know any good ones?

I like looking at games and seeing how many people got some of the easiest achievements, or ones that appear very quickly. 
 
In Assassin's Creed II , 13,719/13,894 (98.7%) people got the first achievement, which takes about five minutes. 
 
Prince of Persia (2008) had a similar achievement, that is also around 99%. 
 
The craziest to me is Chime, which gave away a 50G achievement for downloading it, because it's proceeds went to charity, and somehow this happened: 2,457/2,474 (99.3%). 17 people didn't get that achievement, how, I do not know.
 
Crazy.