Can't you also buy and sell them to people as well, instead of having to (I would imagine) contact a guy one on one and offer a car to trade? Can you even initiate an actual trade or do you have to gift a car to him and hope that he holds up his end of the deal? Can you look at a player's garage in GT5 to see what cars they have?
That's beside the point though. That track editor is a joke, night racing is awesome I'll admit, the weather is sick, but that, Karts, WRC and Nascar aren't particularly hard to do (especially since they didn't have to throw in a ton of realistic damage) and they're not even fully fleshed out modes. Much like throwing in 800 standard cars of sub-par graphical quality, it seems like they've looked away from some of the more important aspects of the game in order to stuff as much as they could into the game.
I feel like in the 5 years that it's been in development (apparently under the impression that no other game had come out since then and added anything to the genre), Forza has moved past it. And I say that as a guy who was, for a while, firmly in the "**** Forza" camp.