In a professional context it is essential that people can express disagreement provided they write exact reasons why they disagree.

You know, "+1" usually has only one meaning:

   - "I agree with whatever you're saying".

"-1" has however two separate meanings:

   - "I disagree with whatever you're saying because I believe that blah-blah-blah (reasons here)"

   - "I disagree with whatever you're saying because fuck you that's why"

And "-1" button doesn't distinguish between those two cases.

You know, "+1" usually has only one meaning:

Nonsense.

    - Agreement: "I agree with whatever you're saying" 
    - Contributory : "I don't agree with what you're saying, but you elaborated your point *very* well and contributed to the discussion"
    - Mercy: "I don't agree with what you're saying, but the hivemind punished you for no good reason, have an upvote"
    - Popularity: "I like you and will +1 anything you write"
    - Thanks: "I like what you did even if this post isn't relevant"

The second and third one are very common on sites like HN, certain places on Reddit, and Stack*

Downvotes have:

    - Disagreement: "I disagree with what you're saying"
    - Anger: "Fuck you, have a downvote"
    - Doesn't contribute: "I might agree, but you're not helping the discussion"
    - Incorrect: "This isn't about agreement, your advice is wrong and/or harmful"

"Popularity" and "Thanks" happen, those aren't very strong emotions. Sure people +1 post or two, but it wears off quickly.

"Mercy" is a very popular way to counteract -1's, but it disappears if we get rid of the ability to minus a post.

I believe "Contributory" is very rare as is. Hard to distinguish it from "Agreement" though.

So, how about tagging individual comments and then voting for the tags?

I like this idea, even having preset tags ala slashdot.

Always maintained that this would make Reddit a much nicer place. Implement the vote tags, and then give people the ability to sort by votes, filtering out votes for disagreement.

If somebody gives your comment a -1 without a reason, and you are of the opinion that disagreement is only valid given a precise argument - can't you just ignore the -1's?

I'm not sure why this is such an emotional topic. Maybe I just have thick skin, but some random stranger on the internet giving me a down vote is not really even going to give me a moment of pause. Let alone so much that I feel that I have to demand rationalization from every person that cares to disagree with me.