I'm all about realism when it comes to simulation games.
Graphically wise, for LFS I'd really like to see track reflect, even pavement has a shine to when the sun hits it.
Would also like to see a option for a time, so you can set server times. One where it is currently the sun position never changes on the track, one where you could set the rate in which the sun rises and sets. One where you have real time, where the sun rising and setting would take actual 24 hours to do. This would require car headlights.
I to would like a weather effect, but would like different types of weather effects. So lets say it is going to rain. You can even have it from start to finish of a race it is a constant weather, be it sunshine and clear skies, or down pour of rain. Have to ability that the weather could change during a race, this would be great for longer races. Since you may start a race where it is nice and sunny but by the time it finishes it might have start raining. Or have a race that was just a light rain shower that turns in to a heavy shower. So give servers the option of being able to have some weather choice other then on and off. This would also make it possible to have to use both dry and wet tires during one race.
Air and track temperatures to be modeled in to LFS. Types of tires, wet weather, dry, slick, groved slick, but only on the type of car that would use those tires.
Parts breaking away from cars when they wreck, I've hit my F1 car hard in a crash which in real life I'd see my aero package skidding down the track without me. Or that should have seen my tire bouncing down the tarmac.
The ability to have failures to your cars. Maybe hitting the bumpers in turns to hard, you have a chance that your suspension is going to give way and brake, shift gears wildly you may break your transmission. Blown tires, that literally give way, not just go flat.