I agree that it's not really possible but that's what they are asking, haha. Since we have to review the contributions and give them a score, we can't really leave it to the project owners to decide the significance (obviously if it's a task request, then yes). I also think it's easier to judge the amount of work it would take for an average developer to complete something, than it is to rate the significance.
Let me put it this way: what do you think we should put the most emphasis on when scoring a contribution and why? Currently it's the amount of work, the significance and the quality of the code. I think they want to remove the question about the amount of work as I've mentioned before, and since it's not really possible to judge the significance with objective metrics, all it leaves is the quality of the code.
All of this has left me pretty confused, to be honest. I'll guess I'll ask during the weekly tomorrow and see what they have to say - maybe that will clarify a few things.