What I worry about is the shiny achievement window that will pop up with a "fancy" sound in the middle of a dark and scary dungeon and ruin the mood.
If Blizzard adds achievements to the game, please consider making them subtle enough so it wont ruin the experience. Like a hidden achievement window that you can open when you want to that wont make any annoying sounds or flashes, but that will keep track on new and old achievements you have made.
Comparing of these achievements with other players will hopefully take place outside of the game, in battle.net 2.0. Like the win/loss ratio system in warcraft 3 or starcraft.