The ability to simulate very heavy rain on the windscreen making the runway just a blur in that a decision to go-around or not can be made depending on height perception. The ancient Rediffusion 737-300 simulator I work with has a vague blurring of runway lights (night scene available only) when rain is selected on at the instructor station. But it is dead easy to land. The runway lights then become sharply in focus when the wipers are switched on. This is not realistic. In real life, blinding rain can prevent the pilot from seeing through the windscreen forcing (hopefully) a go-around on very late final. Very useful for simulator training.
Ability to make very loud compressor stalls rather than a vague popping noise hardly heard over general air and engine noise.
The ability on the instructor station to observe brake temperatures in those aircraft (B737) that do not have brake temperature gauges. The 737 simulators I operate have no way of measuring brake temps after continuous braking and it is often difficult to convince a student that his braking technique on landing is too harsh with resulting hot brakes.
Visual scene availibility to see bird(s) rising from the runway and hitting an engine. Secondary availibility to have choice of a flame out or severe damage or compressor stall as the bird hits.
Ability to programme the length of a runway in order to permit practice at limiting runway length landings (suggest 4000 ft to 5500 ft for 737). The simulators I work have major airport runways never less than 7000 feet and this often results in landing speed complacency (too fast over the fence)
Wider visual scene in azimuth to enable realistic circling approaches without being forced to "cheat" by programming waypoints around a circling approach simply because you cannot see over your shoulder after passing abeam the downwind threshold.