Utter rot, you can join some servers without owning the car if you are in multi group classes like GT.
Only yesterday I joined a server with new Audi R8 and McLaren 650 and I do not own those cars, yet was able to race against them using cars I did own. But obviously if you do not own the TT you can't race in a server purely for them, but if you own the 919 you can race against Audi hybrids if you don't own them. I know because I have done it umpteen times. And I have gone into drift servers and seen the Japanese cars being used while I was using a Beemer that was free to me.
The DLC, one is based on drifting and tuner cars, quite a few DLC packs are largely road car based, albeit supercars, the Porsche pack is about a 60/40 split, ready to race fair enough, Ferrari packs, again a split. So coming from RACE 07 you would want racing cars largely, even they released road based content that no one ever used online much, other than stuff like Caterhams and Radicals.
A lot of people are not comfortable using mods, so you can't really classify that, and to find servers using them that are open is very rare, you might find the F3 mod being used a bit, but not that many.
Depends on your outlook. Seems a few folk on here are happy to never race online, to use the game as a track day tool. Fair enough. But the guy who started the post did not seem like that, he played RACE07, which for years had several good servers with quick races, that sped through various tracks sequentially at the same time during a night, so you could stay in the same server and race at multiple venues while being in there, helped keep it full and also encouraged you to stay.
AC is rarely like that. It is the same tracks every day, same servers.
And not everyone wants to be in leagues or clubs, some of us don't want that hassle or regimented way of playing. I never have and never will. Being punted is annoying in public servers, but in all honesty part of the game online, do unto others as is done unto you. If you don't want it, race in leagues, if you don't mind dishing out what is given, race in opens. each to their own.