I'm a little late to the thread, but I thought I'd chuck in my tuppence worth....
I was a GPL addict back in the day, played nothing else for years. It was amazing, most realistic thing at the time. From 1998 to 2000ish (my most solid GPL years) it was unmodded, as the community hadn't figured it out yet. The community was solid, everyone racing on the same 11 tracks. You could join a league and instantly be competitive, as you knew the tracks inside out already.
I've tried rFactor, and, weirdly enough found it very restricting. There are SO MANY mods, it's unreal. Completely putting aside the "realism" or "general crapness" of many user made mods, the fact that there are so many thousands of track/car combinations means that you can never stay in one place. Excellent mod (a) comes out, and the new track (b) is the perfect setting for it, great. Spend a couple of weeks setting the car up, shaving those last tenths off your lap time, and by then the community has moved on to the next and new "Bestest Mod Ever". So you never end up racing - which, after all is what this is all about.
LFS on the other hand has only a handful of cars, a handful of tracks/layouts, and is IMO the perfect mix. There are enough track and car combinations to keep the interest up, but not so many that the community is fragmented. There's always some server or other racing your current preferred combo at any one time, and you can get some actual racing done!