Weird. I've always seen the ghost cars of friends in Horizon's Rivals mode. I wonder if you have to hit a certain number on the leaderboard for it to upload your ghost replay for others to see?
Also, you failed to mention Horizon's lack of traffic in online multiplayer, Garnett. Point to point street races and free roam aren't just the same when there's only the max 8 real players and you aren't weaving around traffic like the single player mode. The world is almost entirely empty.
There's more than just races with the playground modes as well. King is a lot of fun, Infection is okay, but Cat & Mouse just isn't the same with the arcade style physics that bounce cars away upon contact. PIT maneuvers that were natural in the past Forza proper games are nigh impossible to pull off in Horizon.