In case you missed the reveal at Blizzcon 2011, MMO-Champion has a straightforward review of Pet Battles.
Keep in mind that even though that all of this was officially announced by Blizzard, there are still no guarantees that everything listed will go live with the next expansion. Things could change between now and the release of MoP.
Also keep in mind that there are still MANY MANY things that we just don't have enough information about yet. So unless it's listed on this review page or stated by Blizzard elsewhere, everything else is speculation for now and we really need to just wait and see how things play out.
The features that I hope make it into the live version of MoP even if everything else doesn't: Account wide pets and unique player-given names for companions! Yes please! :D
As for Pet Battles in general... well I'm still really torn about it. Part of me is curious and would definitely try it out, while another part of me is somewhat resistant. The things that I worry about are how this new feature will affect the pet collecting community, pet collecting gameplay, and vanity pets overall.
It was stated that it would be entirely optional, but it was also mentioned that Pet Battles would "unlock countless more pets". I'm the type of person that doesn't enjoy being pigeonholed into a situation where there's only one route to take. I like my options (but not too many as that would get overwhelming) and I enjoy having some sense of control over my style of gameplay.
So IF participating in Pet Battles is the only way to access these "countless more pets" then I don't see how it would be considered "optional" content for a pet collector. If a collector wanted to obtain and unlock the new companions, what choice would they have? If this turns out to be the case, it would be more of a pet peeve than pet perk for me. We'll see.
In any case, MoP will definitely bring about some changes for pet collectors and it's just a matter of time before more concrete details are revealed to us.