Oh there's many little things... damage, race setup, cockpits, brake squeal, setup options, etcpp. NKP for example also like no other somehow manages to convey that you're driving a mechanical thing, as opposed to LFS which is rather sterile and arcade in some ways. It's kinda hard to pinpoint, more like a general overall feeling.
If I had to name one thing, it would probably be the lack of proper consequence to bad driving, i.e. damage. I think implementing that would (hopefully) lead to a big change in how people drive, which is currently the biggest immersion killer in LFS (kerb hopping anyone?). Second to that would probably be setups being restricted to real life constraints, removing another good chunk of weirdness and semi-exploits.