Someone said sound which is really important and worth saying again.

Also from experience: animation and also acceleration/acceleration curves. You want your input to connect logically to what you're looking at so your brain can give you the illusion of resistance and feedback.

If everything just moves instantly or at a constant velocity it feels wrong. There needs to be weight, a sense of wind up and momentum that corresponds with the action and it's surprisingly not too hard to implement. Just remembering that something has to accelerate against inirtia for animations and movement really helps establish that.

Edit: yeah looking at Singularity like you mentioned: I'd be curious to see what you get by lower player movement by a tenth, making the gun animations a little bit more meaty sounding, and stop having all the animation look so instant. It just looks too fast and choppy to me like a pixilated line that needs anti-ailiasing. Just my thoughts anyways.