This could perhaps be improved. It seems to employ a very primitive negative/positive, large/small dichotomy. I presidct that simply putting the words huge and new will give this comment a positive sentiment....
When comments like 'I'm pretty stoked about it actually.' yield a -50% sentiment, something is broken.
Which one?
text-processing.com[1] has a nice api, the bonus was that they restrict API calls on a per-IP basis. So client-side extensions work well here. :)
(meta: Let's see how much a smiley affects the positivity...)