Bashiok: Steam achievements are generally based around small, situational, skill, or long-term goals in short "round" or level based FPS'. So obviously achievements in an MMO are going to be vastly different, and can cover a much wider range of goals from small and fun, to long and involved, and yes even to the impossible.
For us completionists we're going to be frustrated by not being able to have everything. But part of the system is specifically to show off where you've been, how long you've been playing, and what you've accomplished. I'm going to have to deal with the fact that I wasn't everywhere or did everything at some point in time. I'm going to have to focus on the goals I can achieve instead of those I can't. There's no point crying over milk that was spilled two years ago that I just found all dried and crusty under my refrigerator.
That's World of Warcraft though. It's a persistent world and moves and evolves and changes with patch to patch, and expansion to expansion. There's a lot more area for there to be achievements that were attainable at one time, but due to that evolution of the game - are no longer possible.
Diablo III isn't an MMO, it doesn't evolve in the same way, and so there's far less chance for there to be an achievement rendered impossible due to game updates.