overall feel of the cars, I guess... I think 'sim' has a pretty high standard for realistic weight, collision, etc... and in a lot of those areas, Driveclub isn't very realistic. it considers those attributes, which a lot of arcade racers don't. but it's like... you have sort of old school NFS controls, maybe something like NFS Hot Pursuit 2 (the PS2 one that came out alongside Burnout 2)... it's not a drifter, and the cars try to feel more realistic. but the controls are just forgiving enough that it doesn't feel like a sim. DC is kinda like that, too.
it's a thin line tho, for sure... a lot of old topics I'd argued for DC's kinda sim-esque elements. it's just, it's commonly accepted as an arcade racer and at this point it's not a hill I wanna die on ;p and I think it's just a nice blend of the two... still an arcade racer but with sim elements, such as the added importance of car weight, momentum or inertia from elevation change or breaking (changing the center of gravity of the car... impacting your control, based on RWD, AWD, etc). it considers a lot of sim variables but with a more forgiving arcade-y core feel.
could be why it was hit or miss with a lot of ppl... maybe not enough of a drifter for arcade racer purists, but also why a lot of sim fans said it had shitty arcade controls haha. too much of a hybrid for either camp, I guess.