Not a fan of the art design on the Covenant. They just appeared more professional in Reach. They look like a bunch of militia in H4, is what I always feel (which I guess they may be, but still not a fan).
Art design on mp spartans. I hate the mesh design (and accompanying color shift). Noble team and Reach spartans looked like walking tanks - the S-4's look like guys wearing plastic armor attachments.
Related to above, but H4's million armor permutations seem far less consequential than Reach's were. In Reach I was often specifically gunning for a particular look or armor set - here I'm kinda like "meh" when I get an unlock.
Grenades were not "floaty". It really bugs me how slowly they come out. Like an underhanded granny toss or something.
Explosions can be very underwhelming in H4 for some reason. Compare the destruction of the UNSC Savannah in Reach to the [SPOILERS] Covie cruiser in H4's first level.
Invasion mode was awesome.
Covenant competence. In Reach the Covies were simply tougher. In H4, you have to turn on Thunderstorm on Heroic for the elites to actually have a sense of self-preservation - otherwise they will sit there and just take punishment to the face without trying to evade or anything. The Covenant AI just doesn't seem as tight. I don't fear the Covies, even on Legendary, unless they have a fuel rod/instant win cannon.
All that said, I still love Halo 4. I just think there are some things Reach did demonstrably better.