Well I don't know.
So, I just think it's the mood of the game. I mean the environment graphics and visuals are by far the best for any Halo game, save for the new Covenant design which I also don't like that much.
But yeah, the levels felt dull, empty, repetitive, more so than any other Halo game. Unlike the older games (Sgt. Johnson, 343, Arbiter/covenant characters), the characters in Halo 4 were totally uninteresting and completely unfamiliar. I didn't care for any of them. I read some of the newer Halo books (about the Didact), but his appearance in the game did not remind me at all of what I thought he would be like. The MC/Cortana interaction is very different, and their dialog is annoying at best.
The Promethean enemies seem random and combat with them does not develop. You are introduced to the three types upon the first encounter, and their tactics never change. The dogs are annoying. The Promethean weapons are also simply reskins of equivalent human and Covenant weapons (i.e., light rifle, DMR, and carbine are pretty much the same thing, same thing with suppressor, assault rifle, ion gun thing). I also found myself constantly running low on ammo or having to scavenge weapons? More difficult? Not really, just more annoying.
I don't know. Just shitty and doesn't feel quite like what a Halo game should feel like. Campaign-wise, I agree, Halo 1 and Reach were the best, followed by Halo 2, then Halo 3 a distant last. At least Halo 3 followed onto a world that we cared about and was well developed (i.e., Convenant invasion of Earth, firing of Halo rings). Halo 4 story seems like a fanfic, and is not even close.
I agree, the best part of the game was the starfighter part.
I'm just disappointed. People shit on the Halo story, but I've enjoyed each of the campaigns until Halo 4.