Its a World racing sim - i.e. not focused on local cars tracks but on fictional cars and real cars that have a wide audience on fictional tracks. I have raced and made friends with people from every corner of the Earth. (improvements in collision physics and the recent language updates will make this even more true)
Its very Online oriented - has very good netcode (started out with 56k modem back in '04) very easy to find online servers, great facilities for sharing sets, chatting, etc Excellent support for addon apps to improve online server functionality and LFS functionality itself.
Physics - the physics along with the community is what first attracted me to LFS (and still does) It has great physics which are being developed further as time goes on.
The Community - I also don't just mean on the forums, although I've had many interesting discusions on the forum the friends I've made are almost always online first.
It amazes me how many old timers have never posted on the forums
and it's a loss to the forums that they don't as many are great gentleman who are always a pleasure to bump into online.
Also it's a great community in regards to how much alot of the members cotribute past and present, this has certainly increased my enjoyment of LFS.
Lastly it's a great community because the developers interact with it quite openly.
The development process - I actually thoroughly enjoy being apart of the process, testing each new patch participating in discussion when I have something relevant to say. Seeing how each new development effort changes the behaviour of the cars/online experience, etc.