You probably should not expect to just waltz in and get your whole raid outfitted in matching black drakes. The mount drops one at a time, and the drop rate is pretty low. Think pre-nerf Baron Rivendares Deathcharger mount low In addition, the mount will have an equal chance to drop from both encounters, so you have a chance at it no matter what your preferred raid size. It sounds like Zarhym has not ruled out the possibility of an old-fashioned hard mode achievement depending on how you face the fight (I am fully expecting a Many Whelps Left Side style achievement myself), and we already know the new Onyxia is permanent.
All three accounts are pretty bare at the moment aside from some catchphrases, but I am going to bet the accounts will be used as another miniature news outlet, much like the front pages of their respective websites. Considering Kat Hunters job in the company, it is probably also safe to say we will be seeing a lot of Runescape Gold and tournament plugs. If you are a Twitter fiend, you might want to click that follow button for them (and our account, too). If you do not normally use Twitter, it is too early to say if these accounts are anything you should go out of your way to check out. If there is any major news showing up there, it will find its way here too.