It's hard to describe as I think there's a ton going on behind the scenes that we simply don't know. You really do have to try it, but be warned, it's wildly addicting.
Well, I did try the beta briefly, and will have the full game in a few days, but I think I have a pretty good grasp on what it entails as of right now tbh.
I don't envision there being a lot of invisible complexity in the system. The safety rating is just an alternative skill value that's affected by collisions and out-of-bounds, much in the way that a dirty lap would be in Forza currently. There's likely some additional considerations in regards to assigning blame (though it seemingly just treats both players as guilty, assuming this will balance out by the actual guilty party getting involved in more collisions over time), but at the end of the day, you're just ranking up and down based, and then adding this consideration into the matchmaking along with others such as wins/losses, region, etc. Again, I'd liken it to the Trueskill matchmaking that Halo 2 popularised. There's work to be done sure... but not game (and specifically content) altering work.
If GTS wasn't decided to be an "online focused" game, then are we expecting they would as of right now have multiple hundreds of cars, and tens of tracks ready, in line with other games releasing in the same period? How is any part of the Sport mode responsible for that? How does an online focus result in less content to play online with?
I don't think the universally praised online is the only thing they worked on. Assets need to be rebuilt for the PS4, as well as a graphical and audio rehaul. They also have a smaller staff size and did not want to outsource.
Well, that's why I picked Forza 5, as it was the game that was built from the ground up in 2 years for the start of a new generation. Graphics, audio, physics, all of it are changed significantly across iterations in all of these games, online focus or not. They're not things that an online focus changes, and so they shouldn't really be considered. The online focus necessitated the SR system, and the timed event in matchmaking. There's no other part of the game that's specific to it.