I'm a bit of an achievement whore.... I've noticed that there are achievements linked in the game.
What is the total number of achievement points that you can earn in Diablo 3?
There is a total of 450 achievements (including Feats of Strength) in Diablo III (currently, they might add more later in expansions).
Feats of Strength give you 0 points and there are 9 of them. Twelve achievements give you 20 points each. The rest (429) gives you 10 points each.
9 * 0 + 20 * 12 + 10 * 429 = 4530 points in total.
Out of the 4530 points, however, there are class specific achievements which obviously you can't all get on one class.
The achievements are shared across all your characters on the same account, so you don't have to get them again when you create a new char (unlike WoW, mumbling....)