In Mists of Pandaria, many of your achievements and all mounts are going to become account-wide.
This means that you no longer have to complete all quests in various zones for your Loremaster title.
You will simply have it on all of your characters, once you complete it on your main.
Here's the achievement tab- the account-wide achievements are always marked with a blue stripe.
There we have the Superior, Mystically Epic, Mystically Superior achievement. Notice that The Mountain o'Mounts achievement, is marked with a blue stripe, hence, account-wide.
The mounts you get from completing meta-achievements are now also available on all of your characters. This means you no longer have to complete the Ulduar meta, in order to get your Rusted/Ironbound Proto-drake multiple times.
You complete it once and that's it.
Here's a quick demonstration. I've created a level 1 human monk and as you look at the achievements tab, the achievements have not disappeared, even by switching from horde to an alliance toon.
Pets are available from the get-go, but for the mounts to unlock you need to be level 20, until then, they're simply greyed out.