Lore purists, brace yourselves. Right this second, my righteous Paladin is riding the evil demonic steed of my Warlock in World of Warcraft's Mists of Pandaria beta--and he's loving it.
Last night, Blizzard made mounts account-wide on MoP beta servers, which means that you can ride any mount you've collected, no matter which of your characters originally collected it.
There only seems to be one restriction so far: faction-specific mounts like the gryphon flying mounts Alliance players have or the Great Exarch's Elekk that Draenei Paladins get are restricted to Alliance characters only. Shared mounts like the Brewfest Kodo carry over between all characters, however. There are also reports that flying a mount that has a higher flight training skill required than your character currently has will auto-train the higher flight skill. This is most likely a bug, but if it carries over to live servers when the game launches, could save you a ton of gold in flight training costs.
Mounts now have a new UI, similar to the Pet Journal added in MoP for vanity pets. If you're in the beta, you can find mounts by clicking the new button on the toolbar that has a horse on it, rather than a sub-menu within the spell book.
What mount are you most excited to share with your alts?