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.
I wasn't sure on this, but I didn't think it was straight 1 point so the 0.5 might be right, I am sure other people have more evidence.
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!!)
My bet is on 1 and/or 3.