How would you know? How would you know it does not stiffle unpopular opinions?
I think that I would personally practice self-censoring if I would lose karma by expressing unpopular opinions that are likely to be down-voted. I know that I already do on reddit, and there the down-votes are at least only temporary.
Not wanting to lose karma may seem a foolish reason for not expressing your opinions, and at first I din't mind. I kept writing my unpopular opinions on reddit, the down-votes made me put even more effort into them so that they would be above approach. But they' still only get a handful of points whereas one-liners expressing the popular opinion would get much more. So eventually I gave up.
Now I mostly just read a couple of stories a day, and occassionally write the one-liner comment.
As a final comment, I've always been disgusting by the way that people on reddit would constantly compare themselves, always favorable, to their bigger, would-be rival: Digg. Now you do the same thing towards reddit, and I can only shake my head in disgust. Learn to value your site in itself, not as the compare to other sites. It does not need to be a competition.