Simple answer - Submissions are more important than Comments for News.YC ;-).
If comments were given more weight tending towards that of submissions, people would have trouble moving onto the next submission, in the process of accumulating karma. I am sure YC wants people to add value through new/novel submissions.
Sorry about that aston and immad. Even I believe its not 0.5 deterministically, but I think it averages to 0.5. The system appears far more adaptive/dynamic than a completely deterministic model as suggested by the formula. But again, its a model.
My friend, up-voted me once each for two separate comments, but my Karma remained the same. As a matter of fact, I observed something similar with the submissions as well. This could imply the following -
1. Karma calculation is not interrupt driven, but its timeout based (I probably hadn't waited long enough!) OR
2. The weight for the comments (/submissions?) could be a function of user Karma, with an average of 0.5 (I probably did wait long enough!) OR
3. My friend's up-voting was suspicious (Highly doubt it!!)
Errata - My friend up-voted me on two different comments, once each. My score should have increased by 1 Karma point going by the 0.5 weight for comments, but it did not, immediately.
If comments were given more weight tending towards that of submissions, people would have trouble moving onto the next submission, in the process of accumulating karma. I am sure YC wants people to add value through new/novel submissions.