have done a quick read around and could only find this so far:
Originally Posted by Ghost Crawler
Some achievements are "meta achievements."
These are achievements that require you to get other specific achievements. An infamous example is "What a Long, Strange Trip It's Been." These achievements generally ARE account-wide. If one character earned Noble Gardener and another character earned The Flame Warden, then your account would get Long Strange Trip. (Otherwise it would be weird: it would look as if you had the sub-achievements, but you wouldn't have the meta-achievement). Even though you need Explore Thousand Needles on one character, all of your characters can contribute to Explore Kalimdor.
I'm planning on finishing off the original glory of the hero meta achievement - does it matter what toon i do the remaining achievements on?
also as an aside - for the amber void, emerald void and ruby void achievements - do the drakes scale with level, i.e. can i solo it? or will i still need a group?
For meta achievement like the glory of the hero it does not matter which character you do the remaining achievements on. All the achievements are shared between your characters. Achievements like [Lockdown!] do have to be completed on 1 character. Achievements like this will not share the progress.
As for the void achievements. The drakes scale with ilvl, so this should be soloable by now (maybe you can even solo the boss without using a drake? ). Doing this with 1 or 2 extra people makes it a lot easier though.