http://us.battle.net/wow/en/forum/topic/9344554603#1There is a difference between removing a reward without advance notice, which is something we do indeed want to avoid doing, and introducing rewards that are stated up-front as being limited-time-availability (e.g., Gladiator mounts, the Kor'kron War Wolf, WoW Anniversary companion pets, etc.).
We want to strike a balance between permanently available rewards that encourage players to go back and revisit content at a later date, and carving out some things that signify an accomplishment in a particular time and place. Years from now, when people see you riding a Kor'kron War Wolf, they'll know that you defeated Garrosh when he was the biggest threat to the safety of Azeroth. There's something to be said for that.
There will be two other mounts from the Siege of Orgrimmar raid that will be available via more conventional means: First, there's the Reins of Galakras from the "Glory of the Orgrimmar Raider" achievement, which will no longer require Heroic boss kills, and thus be available to any interested players who can pass the various skill checks involved, on Flexible mode or above. (This also lets Heroic raiders who want the achievements or associated mount pursue those goals without compromising their Heroic progression or gear needs, as many of our "kill the boss in an unusual way" achievements have unfortunately tended to be nearly impossible to do on Heroic while the content is relevant.)
Second, there will be a separate 100% drop mount from Heroic Garrosh, which will eventually be reduced to ~2% once the fight can be overpowered, in the same vein as Invincible or Mimiron's Head.
For those who didn't fully read the blue/the original post
Normal mode end boss= Wolf mount that garrosh rides
Glory of the SOO=proto drake that some horde chick rides around org right now I believe.
Heroic Garrosh=???? so far but confirmed 100% drop rate that will be lowered to 1-2% in 6.0
I think its kinda dumb to remove heroics from glory glory but meh maybe ill finally get around to doing a glory achievement since we usually don't even do the achievements after heroics cause most of the glory mounts have looked like shit. But hopefully this makes blizz now make a 100% heroic only end boss mount every tier. Also there will be a normal mode mount for ahead of the curve achievement kinda like t11 i believe it was? thoughts?