Another is to gray down comments with negative scores to near invisibility, in the hope that this will cause downvotes to tail off.
I'm not sure how long this has been going on but it's really annoying. I end up having to highlight the text to read it. The methods of hiding unpopular comments that Reddit and Digg use are much better because they provide the option to view the comment unobstructed if desired.
I think it signifies a trust in user votes; basically, if you hit a certain negative votes, the odds of you wanting to see this comment is thought of as low... having the system this way builds trust among fellow members of the commmunity (hope I'm not way off here)
Firstly, when I see an extremely light gray comment, I'm always curious to see what could possibly have been so ridiculous as to justify that kind of downmodding, so I'm more likely to read it than I am a positive-karma comment.
Secondly, if it truly was a stupid or objectionable comment, it gives some satisfaction to downvote it, and watch it turn an even lighter shade of gray.
So for me at least, this feature actually encourages the two things it's supposed to discourage: reading negative-karma comments and downvoting them to oblivion.
Perhaps this is related to the gamma of your monitor? I understand that Macs use a different gamma (1.8) than PCs (2.2), which would make bright tones calibrated for a PC less distinguishable on a Mac. I'm using a PC, and I have yet to see a comment I needed to highlight to read.
I'm not sure how long this has been going on but it's really annoying. I end up having to highlight the text to read it. The methods of hiding unpopular comments that Reddit and Digg use are much better because they provide the option to view the comment unobstructed if desired.