I'm not sure why that would be an issue to have not implemented at least a 150 achievement already ? With account wide mounts, it actually also increases the likelihood that an account will have more cumulative mounts that before, due to people playing multiple characters and having mounts on alts that they couldn't obtain on their mains. That only reinforces the reason why the number of mounts per account will be greater after 5.0.4 releases. For example I have Zulian Tiger, and Fiery Warhorse on an alt, which I could never get on my main. So in 5.0.4 I will gain mounts cumulatively through this change. Is this a reason to not put the achievement in ?
As an observation, on Beta I saw an Alliance player on a Horde Dragonhawk. That's great, I don't object. It's another mount, they levelled characters on both factions, and probably 100 mounts on both sides, Kudos to them. Why is it people putting effort into something in contrast to someone who hasn't, means don't reward the people whom put the time in ?
I also don't disagree that with the Account Wide Mounts situation that some mounts should only be usable by their owner. I do however think that it should count towards the account's mount TOTAL. For instance, I once had tailoring and made all the carpets, I am now and Engineer. I have those mounts also. My mount total includes carpets, but I cannot use them. That's fine by me. It's inclusive, and yet only if my tailoring alts on my account will be able to use them. That sounds fair and balanced. Just like my engineering alts can use my main's engineering mounts and so on.