No - if I was to be definite. I think it would just make it easier to spread negativity. If people are going to be negative -then make them work for it - make them put their words down. This not only makes them more accountable for their negativity, but also can encourage debate. A 'thumbs down' is too vague/generic. Also - some people can offer creative criticism, which is totally different from others that only want to troll.
LOVE the emoticon idea tho
- some comments are incredibly witty - while other ones give great wisdom, - and some can be very caring. The emoticons would be a great way to show 'why' you liked someones comment.