Here's how I'd put it: there's two ways Halo has changed, both of which are sort of the inverse of each-other. First, there's the plot and story style - which is what you were talking about - that's gotten much more bright, flashy, and consequently, kind of stupid. Second, there's the actual aesthetics. By this, I mean the feeling you get from the in-game environment, mainly from the way things look and sound. This has gone the opposite direction: over-the-top grittiness, with a dark feel more reminiscient of StarCraft or the various apocalypse and post-apocalypse shooters floating around (excluding Destiny), whereas the original Halo had a light, fun feel that drew you in and made you go "Yeah, those Grunts rock" or "Yeah, this level is awesome". I find that both of these are bad things, perhaps partly because they clash, but mainly because they're just the wrong ends of the spectrum to go for.

For instance, if the plot and story style still felt like more classic sci-fi, with emphasis on the Flood, the technological elements, and so on, then it would fit the gritty aesthetics they're trying to capture. However, do we really need gritty aesthetics? Or did the original Halo get it right by giving you more fun and engrossing aesthetics that pull you into the world while still presenting hard-core sci-fi lovers with a very sci-fi story? I would think the latter. As is, there's a slight clash in aesthetics and story styles, but for the most part, they're just aiming for the wrong things in both aspects.

For a comparison, just look at where Destiny seems to be headed. Despite technically pulling some tricks out of space fantasy, the story and plot style is very sci-fi-ish, while having an engrossing aesthetic style that pulls you in by making you bask in the awe and wonder of a massive world while also presenting you with some of the cute, fun aesthetic of, say, a Halo 1 Grunt. This is the opposite of where Halo seems to be headed, and before we even know when the game will be released, we can all tell it's working for us.

Vale,
Perseus