Account wide achievements
I'm not sure if it's possible to add account wide achievements but if it is then it's not a bad idea. Not really a custom like feature as blizzard have added it later on.
Icecrown (and probably other realms aswell) is super reliable on having an achievement in order to get into a pug raid. Instead of logging into a main to prove your experience it would be better to just have everything ready to link wherever you're logged on. Also it would be easier to see if a person is lying about an experience or not. Same goes for PVP people trying to find an experienced player.
Overall I think it's a great feature that Blizzard have added in retail.