First and foremost:
"You also won’t be able to use a faction-specific pet, mount, or title on the wrong faction."He does note that there may be some exceptions, but it's still a WIP and the developers will address any issues that pop up as they run into them.
Regardless, this is somewhat disappointing news, but I can see how allowing the use of cross-faction pets would be inappropriate. I guess this means no Horde Balloon for my nelf druid. Pity.
Secondly, Ghostcrawler explains how some achievements will be "account-only". This basically means you could add to the progression of a few achievements across multiple toons. So for an achievement such as Legendary Pet Battler, any pet battles won on any toon would be cumulative and add to your existing total amount. This is just an example, though, and it was mentioned that only a few achievements would behave in such a manner. We'll have to wait and see which ones actually fall under this category.
The system of account-wide achievements is obviously still unfinished, and it will need to be refined further.