The Checkpoint System
Why? I can see that this game has a fundamental need for some mechanic to prevent people just driving in a straight line from one place to another in races, but the checkpoint system is such a bore. The checkpoints are too narrow, and when racing nearby others its easy to end up being pushed or simply hit into a position where you miss the checkpoint. The worst thing is that the checkpoints will even punish you for taking the slower line. Slightly screw up a corner and end up a metre on the outside of a checkpoint? Well prepare for even more punishment.
Why are they -so- punishing? Missing a checkpoint can be utterly lethal to your chances. It takes a couple of seconds for it to start counting down, three more for it to reset you, and then you end up way further back and starting at 0 mph. Online, where people will happily and purposefully push you out of the way, this is absolute nonsense and totally game breaking. It's all very well to advertise the importance of racing clean but it's pretty obvious the game has no idea if you are racing cleanly or not and even more clear that nobody cares one way or another.
I don't think checkpoints are a terrible idea, but there are too many and they are too punishing when you miss them (or are forced to miss them). It would also be nice to see more variety in race types. I loved Burnout Paradise's races where you'd be given a reasonably small number of checkpoints and allowed to take your own route. Forza Horizon's, comparatively, is a tedious constraint to fix the issues created by having a game where there is little bonus to driving on tarmac, and it's not a remotely worthwhile tradeoff.