The problem is that they decided to scrap the amount of work from the guidelines as they didn't deem it important; apparently just rating the significance is enough. This means I'm stuck trying to think of a way to solely judge that, instead of having some sort of combination of volume and significance as we did before, and as you have touched on above.

I think you've brought up some great points about the value of a feature and if the contributor met its goal (especially for task requests). Still, it's pretty hard to come up with an objective way of judging the significance of a feature (especially for own projects) in comparison to the rest of the project (and other projects). I think this kind of stuff will always be subjective no matter how hard we try to use objective metrics, so I am a bit at a loss how to rate this. Especially without taking the amount of work into account.