Couldn't the limitless gameplay possibilities also just as easily give it a 1/10? If that's going to be a valid reason to score it a 10, then I'm assuming we're judging the actual user content, which can range from the amazing to the piss poor. I personally think this game will earn great scores, but not for that reasoning. It'll get the scores for the physics, gameplay, and the accessible tools one can use to create levels with. I don't think the levels themselves will go under scrutiny.
If the user community ends up sucking it should get an algerbraic rating... like = A.
Where A is something like ability to make levels + your esteem level.
So if you are real good at making levels and feel ok about yourself and your abilties, high score.
If you suck at making levels but think they're awesome cause you have a big ego, high score
Good at levels but the over critical type? Low score.