Please use pluses and minuses wisely!
Wisely used pluses makes valuable comments and answers appear on top. On the other hand - giving minuses to silly, bad or misleading comments makes them appear below the valuable ones, or even hides them.
But for heaven's sake don't give minuses to questions asked by people who are learning the language and really don't know something, or are trying to understand, but write wrong. It is the purpose of this site to learn, and learning when someone corrects you is very efficient. Correct them, instead of condemning.
I always thought having a system of plus/minus (thumbs up/thumbs down) doesn't help at all because people can never agree on using it correctly, and there's also people who go comment by comment spamming one of the functions. At least in every site I've seen this being implemented it causes total wreckage unless you pay no mind to it.
Just a "thumbs up" button has always been a better choice, honestly.
From my point of view it works quite well in most cases on Duolingo because comments without any relevant content get out of sight quite quick. What just annoys me most is exactly the same gabe81 said: When people downvote your question/comment instead of telling you that you are wrong and explaining why. But this is just a minority and I for my part make good use of the +/- system.