It's breathtakingly mediocre in my mind. It does nothing unforgivable (besides Chief and Cortana's shallow, dysfunctional high school relationship, perhaps), but it really does nothing to wow me in any of its portions. The single player is whatever with a few cringeworthy moments; the co-op episodes are short and equally forgettable; and the multiplayer is serviceable with only a few balance issues (like the DMR being a hitscan weapon with no recoil or bloom whatsoever, and there being no maps to accommodate close-quarters/arena-style fighting). Every facet of the game is outdone by a previous installment -- I can't think of any portion of Halo 4 I can hold up and proudly proclaim, "It does this particular thing better than any other installment." Oh, and once again: Fuck the maps.
I really do find rampancy to be the biggest let down. There's a lot of potential to be creative with that idea. Even if the idea of a homicidal AI has been done before, you can still expand upon it and add a fresh twist to it, like having Cortana leading Chief into traps or ambushes and playing them off as accidents, to the point where Chief has to come to terms with the fact that he has to get rid of Cortana before she degrades even further and really tries to pull off some kind of a stunt that'll kill them all. I would have genuinely felt bad for Chief once he reached the conclusion that he would have to put her down against her will. Or if we really must have a romantic element to it (I rather despise romance in stories, since they tend to be a cheap and easy way for writers to garner sympathy for characters without having to work for it), have her try to kill everyone around Chief (again, playing them off as accidents) out of jealousy so she can keep Chief to herself (which then leads to a similar moment where he has to grapple with the need to let her go and destroy her). There's just so much potential, so much room to run with the idea, so many avenues to explore with it, and they phoned the whole thing in with a Twilight romance.