The process by which exclusion is achieved, through attainment of Order of Cloud Serpent Dailies (which itself can only be gained through another gated, dailies system), is honestly pretty lazy.
Lazy... as opposed to... what? Grinding reputation via tabard buffed dungeon runs? Defeating a fixed number of dungeon/raid bosses? Acquiring a tremendous number of high end, hard to find, difficult to farm/craft items to repeatedly turn in over and over?
Reputation gains have never been particularly stunning feats of design in terms of complexity.
There's no point in stymying a player's selection of what mount to use in this manner. The hard part of any of the raid/dungeon achievement mount, or the rare mount, is actually getting it in the first place. Adding another restriction on top of that, a restriction that's entirely based on an easily-attainable, but utterly pointless, achievement does very little to advance Blizzard's reasoning for implementing account-wide mounts and achievements.
Easily obtainable doesn't do your argument any credit. It is like talking about how difficult it is to achieve a perfect score on an AP test while also talking about the pointless restriction of getting into an AP course (which merely requires talking to the teacher and filling out a form).
Also, there is a point to it. Exclusivity still factors in and the only way to maintain a system that allows antiquated mounts to be usable by all across all characters but make new and exciting mounts actually somewhat rare is by adding in these restrictions. The big meta achievements ARE indeed noteworthy... hence why some random Outlands alt shouldn't be roaming around on it.
If you're going to allow players account-wide mounts, and players will understandably want to show off the newest (and impliedly more rare) mounts, why implement such a restriction? To give players something to do on their alts? I honestly don't the reasoning behind the "exclusion" process, since it is so easy, but annoying, to get, and is not indicative of any real skill.
Things aren't all or nothing and never will be. That is what isn't understood by most of the posters. Furthermore, if it takes no skill and its just annoying is still not compelling. If its just annoying to you, then it really isn't much of a restriction and we're just talking about antsy stuff (to which I already said: stop whining).
Hypothesizing what would have happened during WotLK and Invincible is dumb, since there's no way to know what would have been had account-wide mounts been in place at that time. Ditto with "well, they might allow account-wide Cloud Serpent riding in the future." Instead of arguing what could have happened, or what may happen, focus on what is happening.
The comparisons to past mounts are perfectly relevant in that you had to actually acquire the mounts a second time. What's happening now is... they're making some mounts exclusive like they have in the past.