I don't know what it is exactly. However today I installed rFactor again, got the new popular mods (Corvette, Historic GT and Caterhams) and a few nice tracks. Pulled the MOMO out of the wardrobe (where its been for 6 months or so) and got everything set up. I've literally only just stopped playing, 8 hours on.
With these user-content based games people always bang on about how a large proportion of mods are bad, but similarly there are a large number of VERY good, entertaining mods out there.
LFS on the other hand, well, I havn't played online for god only knows how long, maybe 18 months? There's just nothing new to keep me playing. 6 months on and we've got some interior updates and more languages. Great. Try sorting the game out first, then look to increase your user base by adding languages.