In anticipation of hackathon (http://www.duolingo.com/comment/774419) I made couple of more suggestions so let me know what do you think about them :)
I'm not a programmer but it think only #1 for discussions could be difficult to implement.
Always show the Duolingo's main translation in lessons. (Sometimes my translations are very literal and if there are no comments in the sentence discussion I forget to check duolingo's main translation)
In listen and type when we make a typo show the translation along with the correct solution. (Lots of times there is blank space where there should be a translation in sentence discussion.)
Ability to follow a certain comment or a button "I want to know this as well." (This one feels a bit redundant since we have "follow discussion" option but If there are a lot of comments and we want to keep track of specific ones this could be helpful.)
Remove down-voting outside of the discussion
Add follow button outside of the discussion
As you said and if I am not mistaken up-voting doesn't provide developers with an information that there is a question inside the discussion that needs attention since it can mean so many different things.
Creating that system would be something even more difficult than what I have imagined which is to have an ability to mark a comment so we can get a specific notification in our notification bell when somebody replies to that question/comment we were interested in.