While making it so you can rate comments that are constructive as “good” may be a good idea, it would definitely be too much for Scratch servers. There still would be terrible, un-constructive comments such as:

“Good project! Check out my project here: ?????????”
“If you love it on my project, I will love yours”
“*enter un-constructive/trolling/flamewar comment*!!!!!1!!!1!!!!1!!”

My idea is to give us templates to choose from, such as:

Blank (normal comment)

Feedback (default)
-box one: compliment/what was good
-box two: constructive criticism

Remix compliment
-box one: A thank you for remixing my project
-box two: What you think was good and what you think should be improved on

So it will be easier to create a constructive comments. While it will not prevent “IF yoU No ChEcK out ME PrOject I No LoVe It” comments completely, it would soon make it more boring to make spam comments as you'll have to switch to the Blank template from a drop down menu every time. It definitely would deter spammers.