I have yet to play a game that took me to an immersive, completely realized universe the way Halo 2 and 3 did. And I've come to realize that the 10,000+ lines of AI dialogue was a huge part of that. Every ally and every enemy was a fully realized character with a past and a future, a culture and a viewpoint. There were marines remembering their buddies on Reach, or inanely commenting that the Brutes smelled like wet dog. There were Brutes not just taunting humans and Elites, but explaining exactly how they would mangle your corpse when they were finished. All this fiction-infused dialogue made those campaigns truly unique, and a different experience every time I play it.
So yeah, I agree entirely that Halo 4 (and to some extent, Reach) felt like an emptier, more superficial world without all that AI characterization. Here's hoping that Halo 5 brings combat dialogue back to its former glory.