So you are basically wanting to play games before you buy them(has nothing to do with preordering), so you know if you will like them before you spend money on them? I highly doubt this will ever happen. If it did happen, companies would spend most of their efforts creating very exciting level 1-10 or whatever the trial content is.(when in reality, that's the least played content as it goes by so quick)
Get back to reality, much like any other purchase in life...
You do a little research to determine if it might be right for you. You make your choice to buy or not buy. Sometimes you are happy, other times it doesn't work out. That's life. Also, change the title, because its not about preordering, its about buying before you play the game(which you would need to do even if you didn't preorder).