One of the big ones for me, and one of my single biggest worries, is AI. I've seen Grunts do flat-out retarded things, including getting stuck on scenery, and Elites barely seem to duck and weave anymore. Saw a Prom dog run endlessly along the same spot on a wall. Knights' teleportation seems almost random. Fights with Prometheans devolve into uninteresting spamming by both sides, and it feels like they've been beefed up to take a lot of damage rather than being legitimately threatening in their own right (except for the Knight melee).
Also disappointed by the weaponry the Prometheans use against you. Against the Covenant, you've always faced lots of differently coloured, slow-moving lasers and plasmas. They're differently coloured so you can identify and prioritize threats almost instinctively; they're slow-moving so you can dodge and juke through the fray and feel like a badass. The Promethean weapons, by contrast, just sort of pepper with you a mess of indistinguishable orange stuff that seems to hit instantly. Fights feel flatter, and some of the early areas you engage Prometheans in are just open spaces with rocks dotted around. So it ends up feeling like a war of attrition where you're trying to pummel them more than they pummel you (Prom dog headshots feel good, but the Boltshot's design means firing single shots feels like it has massive input latency). It risks being graceless at times.
