The main mechanic that this game is missing is proper recoil. And right after that, the "float" mechanic when you run off a ledge. (Most games that feature any sort of jumping or platforms, have a built-in "float" mechanic for jumping, where even if you are 1/4 of a second off a ledge, you can still jump. However in this game it feels like you can't jump even when you are still on the platform, just on the ledge.)
Then there's the missing headshot/critical hit detection. For humanoids, obviously the head would be the prime target, however for the things crawling on the ground, specific parts could be hit for extra damage, instead of aim not mattering at all. Then there's also the fact that Captivity had a specific mechanic for dealing with enemies that are often hidden behind corpses, or generally hard to hit on the ground - You should be able to just walk over the mini-worms to kill them instantly, instead of having to shoot each one.
There's no roll either, despite Captivity's gameplay heavily relying on it. Ammo refills are also not really a thing in this (even something as simple as being able to rebuy the same gun from the wall should be an option).