Ability to build more complex structures (proven by hundreds if not thousands of remaining examples all over the world), and possibly the ability to harvest electricity from the aether.
Discussion
See the first one is entirely down to earth and probably true (though more because they had better education/culture than technology) and the second one is just totally out there.
I don't disagree that we're lacking things which the past had, but those things aren't magical aether technology. They are simple, directly observable thing that our rulers have intentionally deprived us of.
Nice to see we agree on something. And honestly I don't think aether tech is outside the realm of possibility.