"Hey guys! Have you ever wanted to experience all the fun and agony that PC users have had with Steam Greenlight?"
"Well, uh, not really mister!"
"That's great, because here's Xbox Game Preview! You'll get to pay money for incomplete games on the premise of promises that could change the very next day. If the game goes under, well... check out another game!"
EDIT:
Although, to be fair, there is a free trial for every game, which is actually a good idea. No more silly asset flipping like you get on Steam (people making "games" using terrible, generic, Unity-Store-bought assets).