It's a little bit of everything that turns down some people and turn on some others. A sim can't satisfy them all in all areas.
What I'd like to further comment on is about some fellow posters who claimed that it's only natural to get bored if you play the same game over and over again for a vast period of time. It surely is natural, but the whole point of this thread is to seek why this might happening with LFS (correct me if i'm wrong). LFS is not supposed to and does not remain the same game through the years. It's rather an ongoing project, so we shouldn't play the same game over and over (and in some extend we didn't).
So the real question is whether the LFS progress satisfies the majority of the community, especially the online racers according to the topic's title.
Although I agree with Ian that the sim's progress is slow and with questionable priorities, I beleive LFS is still the... king of online racing. I've tried mostly rfactor and GTR2 and none of them offers the same excitement online... not even close! Wheel-to-wheel fights feel really short and somehow fake in those sims. Is it the superior LFS force-feedback, is it the physics engine, is it the fact that I'm by far more experienced in LFS? I dunno, the point is that LFS still offers me the greatest deal of enjoyment online, compared to any other sim I have tried.
That being said, constructive criticism should always be welcome by any sim's devs & community, since it's the only healthy way for a sim to evolve towards the right directions. For me RL tracks and sound rework shoud be high in LFS's to-do list, others prefer physics, more cars and grfx to be done first. Some others prefer rain, night and offline championships. One way or the other, the truth is that LFS is starting to show its (evolving) age and it's up to the devs to get the message early and do something about it, before the competition reaches or even overcomes LFS's current strong points.