Most of the Forerunner structures look great, but I'm really not into the entire circular theme some of the Forerunner structures and interiors have in Halo 4. I mean, I get it because the Halo rings are circular too, but it feels so out of place with Forerunner structures usually having straight lines.
I felt the same with Anniversary where they from this:
To this:
Such a weird looking wheel... thing.
The wheel thing didn't really bother me. The difference from CEA to Halo 4 is remarkably stark, all things considered. Changing everything to a sort of gunmetal and lacquered look along with the use of circular motifs in the level design did feel to me like too much of a step away from the old language established to cement the difference between Human-Forerunner-Covenant. The only parts I really felt at home in Halo 4 were on "Reclaimer" right before you meet the Librarian (while a bit busy it really felt like harkened back to the Library's vaulted corridors) and on Forerunner (the side tunnels reminded me of AotCR's chasm connectors.)
I personally would have preferred a style closer to what we saw for CEA and the Anniversary map packs (love the passages on High Noon) but that's how I feel about most of Halo 4, really.
On the scale of 1-10, with 1 being no change and 10 being Halo 4, I'd rather have been closer to a 5. Keep the stuff we know mostly the same (FUD, Chief's armor, Sentinels and Covies, some of the architectural language) and go for broke on things we don't have a visual language for--I think overall the Prometheans do a good job of looking Forerunner-inspired while still different from what we saw before to emphasize that they aren't the Sentinels.) Likewise the Infinity, the Shotgun, the Railgun all feel like fairly natural evolutions of where we left the UNSC and how they've adapted--the RC Warthog, not so much