On paper, Sport mode is a bunch of simple ideas, the majority of them already seen in other games. What makes the difference is how they made them work as a whole, in my opinion. Quite a lot of people thinks the matchmaking based on DR and SR is just a trick that doesn't work as well as advertised because of people "tanking" their DR to get more wins, rammers in SR:S and stuff like that, but my experience after almost 200 Sport mode races is very positive about the matchmaking and this mode as a whole.
Safety rating and penalties are still far from perfect but we have to consider that it's still a computer which has to figure out how a contact/accident happened, a task which is sometimes hard even for experienced race stewards during real life races. It got better since the launch and it appears that they're trying to improve it constantly (i.e. silly penalties for overtaking crashed cars under yellows seems to be gone, or maybe I've just been lucky the last few days). The same goes for Balance of performance, there are still some over/underpowered cars on certain tracks but again, I have the feeling that they're going in the right direction (especially for GR4), it's nice to see a lot of different cars being competitive in the same race.
About having only 3 daily races, I'm good as it is now, as long as I find myself in a lobby full of people with the same DR and SR as me. Especially going forward, since the player base will likely get thinner as the game gets older. I prefer battling for 13th place alongside similarly skilled drivers in a track/car combination that I dislike rather than racing alone in first or last for the whole race in a car or track that I like. Luckily enough, 95% of the times I join a Sport mode race the lobby is full of identical DR/SR drivers as me: in FIA races there are usually something like 15 drivers in 1 seconds on a 2 minutes lap after that quick qualifying session. It tells a lot about the matchmaking system and that usually translates into really close racing. On the other hand, I've participated in some lobbies with DR going from S to D, and the difference was clear by simply looking at the mini-map, with the field very spread even in 3 laps races. It looked like multi-class racing, except it was one-make Audi TT races. If that's the price to pay to have more variety of daily races, I'm happy with how things are now.
Again about simple things: rolling starts are a good way to reduce accidents at the start, especially because they prevent some people from bumping their way into gaps that simply don't exist and causing mayhem even before approaching the first braking point. Yes, you're still metaphorically bracing yourself every single first corner even in SR:S but thinking about it, that's actually a positive because in most other console racing games you're simply resigned to the fact that as soon as you touch the brakes you're going to be launched somewhere. At least in Sport mode you're left with the doubt.
Another little thing: the chat before and after sessions and its clever, "auto-translated" messages. Again, nothing revolutionary at all. It was already in GT5 7 years ago I believe, but still, how many console racing games nowadays give you this quick opportunity to apologize for a bump, congratulate after a good fight or just let go some of your frustration against a rammer (except for the fact the he always leaves the lobby as quickly as possible)?
One thing I noticed is the very small amount of people quitting races. If it's because the game is pretty harsh with DR/SR penalties or something else, I have no idea. It means you're very rarely left alone on the track, you're always racing someone even after bad mistakes or being smashed off the track. That's a big improvement compared to your "ordinary" race in any other racing game. Rear-ended at first corner? Quit. Bad setup? Quit. First place is too far? Quit. The nearest car is 20 seconds ahead/behind you because everybody quitted? Quit. That's a regular lobby, including GT 5/6/GTS, but it seems different in Sport mode.
Last thing: connection quality; this game is probably the best I've experienced on PS4 regarding that, including non-racing games. Yes, when someone lags it's worse than any other game and it's impossible to stay close to the lagging car, but it happens very rarely and other than those isolated cases the game runs really smoothly.
Having said that, it's not that every race in sport mode is a blast. Sometimes races are dull, sometimes your race gets ruined by rammers or undeserved penalties, car/track combos can be quite boring and some of them are basically unplayable if you care about SR. Also, this mode really highlights the lack of tracks in this game if you exclude ovals and rally courses.
Anyway, all things considered, I'm really enjoying it and at times is huge fun. There's still a lot of room for improvement but as I already said the fact they're constantly improving the game hopefully means that Sport mode will get even better.