so, I have many many toons (so many I'm forced to play on 4 different realms because each realm only takes 11 toons)
usually I don't care much about mounts, but a few days ago I noticed I was close to 150 and decided to buy a few MoP mounts to get the jade kite.
except that due to a bug I didn't get the kite lol
but here's the catch: the toons in my main realm (where I earned the achievement) don't have the kite, but the toons in my 3 alt realms do!
it seems blizzard had to improvise a bit when implementing this accountwide stuff feature. my hypothesis is that when you earn the achievement, the system remembers which toon earned it and sends the mount item to it. mounts are "realmwide" so when you learn them in one toon, you'll get them in all toons in that realm. blizz also gives the same mount to toons in other realms behind the scenes so as it looks like the system shares them with your whole account.
I recognize this is a bit irrelevant, but to me it seemed pretty interesting to catch a glimpse of how the game works