OK. I think the following is the most important thing I have to say.

Halo 4, nay, 343i's Halo feels more fantastical because human agency is dead. Gone. Vaporized. It's dead, Jim.

Bungie's Halo had larger-than-life characters, most centrally a green-armored supersoldier and his purple/blue spirit guide who saved the galaxy. But in every one of the stories Bungie created, it was the characters who saved the galaxy. The Arbiter's defection, the Chief's action, Noble 6's endurance.

In 343i's Halo, everything is the culmination of a plot by some old Forerunner biddy. Also, Humanity can do no wrong. We were the greatest foes the Forerunner ever faced, and our empire once spanned the stars. Our expansion, our effortless crushing of the Elites and thoughtless control over Forerunner artefacts is Manifest Destiny.

Oh, and Elites are now Uruk-hai in space.

: Like, in Halo 1, it was a gritty
: human-kind, fighting against a powerful alien foe, and while a classic
: sci-fi setting, the mystery of Forerunner architecture and looming
: murmurs of ancient civilizations somehow made it feel more..real life.. or
: something like that.

Eh, I kind of disagree.
For me, the most fantastical Halo game (excluding Halo 4) was the original. From Halo: CE, Bungie could have gone anywhere. They could have gone more realistic and down-to-Earth (They did), they could have gone more fantastical (I.E. the Metroid end of the scale) or they could have stayed where they were... which is the part of the scale occupied by Marathon.

I think that, by placing part of Halo 2 on Earth and showing a very Human city, rather than the needle towers of Coruscant or the Hives of WH40K, Bungie firmly established the UNSC as regular humans, not fantasy humans.

: With the introduction of the Didact, and more tangible answers to the
: mysteries of Forerunner existence, it almost feels like we're moving into
: a complete fantasy realm of wild sci-fi content;

The fantastic parts of Halo became almost solely the domain of the Forerunner. The story of the Forerunner's defeat of the Flood tastes like a myth, Beowulf or Gilgamesh. It's a legend of sacrifice and archetypal heroes. It is a tale larger than life, though the story has been worn down and worn through by time itself.

And, now that Halo 4 has bridged the modern day and the time of ancients, that mythic quality has been dragged out back and shot. It was going to happen no matter how they handled it, and tragically they handled it with a senile old man who might qualify as a Marvel C-list villain.

: over-the-top characters
: and designs and action... as opposed to a relatively muted, human-centric
: vision of the future.

Exactly.

: There's less, in my opinion, that's separating Halo now from other science
: fiction stories and universes...

Yes, thank you.
There's a lot in Halo 4 that strikes me as generic and derivative, even though I can't always place the origin.

: Don't get me wrong, I'm not saying I dislike where Halo is going - I still
: love my Haloverse :)... but I think this is one way that Halo has lost (or
: changed) that 'magic' that was around when it first launched; when we
: first discovered the Flood, when we first looked out over the landscape
: that was a Halo ring. Things have just become far too organic, bright,
: colourful, flashy, compared to those moments.

Much like the Abrams Star Trek movies. Somehow, despite omniloathing everything about what Star Trek was, I hate the reboot even worse.