Here is what I miss the most from GT5... Online Shuffle races.
For those of you who don't know what it is, in shuffle races, you did not get to pick your car. The PS3 picked the car for you and all the other drivers in the room within the same performance brackets. The cars were stock, equipped with comfort soft tires or sports hard tires so you didn't end up with "go kart grip". Typically these rooms were always almost full, with great drivers and I always ended up having some epic races on there.
See below a replay of one of those races. Polyphony Digital needs to bring that back to GT6