Um, did you even play Vanilla? Most of what you did was under direction by the Aspects' lieutenants. Did you do nothing for Chromie? Did you not fight any green dragons? Did you not do anything in Blackrock Mountain? Did you even play Vanilla? Do you even go here?
You point it out yourself... Lieutenants... Offspring.. More or less successful experiments... But the Aspects were NOWHERE to be found..
Nefarian, Onyxia and Baron Sablemane are offspring of Deathwing, true enough, but where was DW himself? Absent..
Azurgos in Azshara.. Yeah, he's a blue dragon... But it's NOT Malygos..
4 green dragon world bosses..... No way, you can claim any of them was Ysera..
All in all, the Aspects presence was nowhere to be found, until we killed Malygos, and then completely ignored until we quested in Cataclysm...
What other demigods are involved in the Pandaria story to compare with the Aspects who are present all throughout all iterations of WoW except BC and now Pandaria.
Think about the question you just asked. "What demigods are involved in the Pandaria story." If you're going to compare the AC to any other group outside of Pandaria, then this question is already moot. "What demigods are involved with Azeroth?" That's the issue. If we never met the Dragon Aspects, the Celestials would look powerful, yet controlled, but we've met far more than just them and those we've met dealt with similar issues.
The only time the Dragon Aspects looked even remotely comparable to the Celestials was in Twilight Highlands when Alexstrasza herself couldn't keep up with the waves of Old God-empowered mortals assaulting her children.
There is a definite aura of mystery about them. One thing that has been bugging me is the trash pull before the Twin Consorts in ToT. The Celestial Shapers. They're busy creating images of the August Celestials when we interrupt them. Then they show up again in the Twin Consorts encounter, not as themselves, but as faded images.
This is all just guessing at this point, but these may be glimpses into the actual origins of the Celestials. They might have been created by the mogu in the same way the Aspects were created by the Titans - taking an indigenous creature and empowering it. The mogu did consider themselves successors to the Titans, and did attempt to continue their work, albeit in a way that became perverted and twisted over time. Maybe the Celestials were an early project, done before the mogu had gained a taste for cruelty, or maybe they rebelled with the other created races like the grummles and saurok.
Again, only guesses. They do seem significantly less powerful than the Aspects, but at the same time they use many of the same themes. They're the Big Goods of the expansion, each one personifies a philosophy or concept, they're functionally immortal, and while they aren't capable of quite the same feats the Aspects were before depowering, they are still immensely strong. In any event, they are some of the more intriguing characters on the continent.