Bullet Hit Sounds
It feels like the marines are killing me psionically right now since I don't hear their bullets. Whenever a marine is far away, I'm half dead before I notice I'm being shot at. It makes the game world feel insubstantial, and makes dying confusing and non-exciting.
In ns1, hearing the "ping ping ping" of bullets hitting the metal around you was often startling/terrifying, and the "plat plat plat" of being hit was a much better indicator than anything visual. If I had to pick a single thing to fix about the game right now, this would be it.