Yes, landing on the vast Halo virtually alone was awe-inspiring. But starting in Halo 2, the emphasis the Forerunner's grandeur largely wore off, as it took a back seat to the Human-Covenant war. (After all, you can only play the big-object trick once.) And after Halo 3's Terminals, the Forerunners no longer seemed to have any mystique to me. That they had better versions of technology we already knew much about (and could apply it on larger scales) did not hide the fact that they were really just humans with better toys, locked in a not-particularly-interesting battle against the Flood. Heck, we didn't even have an indication that their culture was drastically different.

At first, 343i did not do this perception any favors with Origins. But to me, the Forerunner saga (and Silentium especially) restored the sense of awe and wonder. By giving us a peek at the Forerunners, we learned that there were questions to ask. Slipspace flakes? Imprinting? The Domain? These are things we still know little about, but know enough to tell us that the Forerunners inhabited a place outside of the modern human experience. And for the "big object / no knowledge" category, we now have the Precursors, which constructed even grander things and lived (and died) even more mysteriously than we thought the Forerunners ever did.

I also don't believe that 343i has removed the human agency of the Master Chief. To start with, he really did not have a lot of it to begin with in Halo. He was practically created to kill for the government on orders. He spends Halo 1 getting caught in one intrigue after another - the Covenant, the Forerunner, the Flood - beyond his control, just doing what he can to contain it. He had more agency in Halos 2 and 3, but never made any decision against orders. (And let's not even go into the Assembly.)

Some people say that since the Librarian "planned" for the Master Chief and Cortana, they're puppets. Instead, I (perhaps optimistically) see the beginning of a classic narrative of the engineered/foretold yet not beholden hero facing a massive choice. It actually reminds me a lot of Dune: the Bene Gesserit may have engineered the Kwisatz Haderach, yet the choices are his alone. And what greater autonomy could there be than that?

This is not to say I think Halo 4 had great storytelling (it did not) or visual continuity (it was only OK, but as I don't primarily consume Halo through the games, that's a non-issue to me). And I do think Halo needs more personal stories like "Second Sunrise over New Mombasa" set in the postwar era, and that realism is part of what makes Halo Halo. But I think that they're making good on their promise to deliver an epic universe.