Quite simply, and might I add poetically, sensationalistic headlines.
We've been exposed to and immersed in social hierarchies our entire lives, so it's no wonder we form similar hierarchies on social news sites. The desire to both fit in and stand out drives people to compete for "karma," recognition, or some other abstract reward for being popular. The best way to be popular is to submit stories with hyperbolic, semi-accurate headlines that appeal to the broadest demographic.
The result is biased news without integrity. True, some stories are of interest primarily to the demographic on the site, but others are explicitly biased.
Yes, the problem with social news is, ironically, the social nature of it.
Now, a solution: since these sites encourage almost capitalistic behavior, there needs to be some large penalty for submitting inaccurate and biased stories--enough of a penalty to discourage such tactics. In contrast, a community (more communist) approach would be to eliminate karma altogether, but people would quickly form black market karma (tm) that's created in the comments section.