A while back Ghostcrawler let drop on Twitter that Blizzard was considering moving the cloud serpent riding to an account-wide unlock, rather than on a per-character basis. Well, it seems like that idea has been shelved. Each character will need to get the achievement in order to use cloud serpents.
Ghostcrawler explains in his tweet:
@Gnomerix I believe I said we could consider it. With all of the catch up mechanisms, it's pretty fast for alts to get there.- Greg Street (@Ghostcrawler) October 3, 2013
This should also serve as a good example to folks of situations where Ghostcrawler talks about something as a possibility on Twitter, and then notes later that it doesn't become a reality. It's important to remember that Blizzard uses Twitter as an informal means of having a discussion about game mechanics -- rarely do they announce anything of importance over the medium (or at least announce it via a non-public relations/marketing channel).
In other words, it's okay to be disappointed that this change didn't make it through (I am) -- but don't say that you were promised it/the pony.