I think what PG called "incompetence" is really just rampant emotionalism. "Incompetence" is inaccurate because it suggests that people of all levels of experience can't happily share a forum, which I think they can. But what happens at Reddit nowadays is that anyone who posts an inane comment with a phrase like "why the fuck" will get lots of upvotes. (Case in point: Search for "fuck" in the comments for the top article at Reddit, and get: "That fucking cunt is going to get what she deserves," 15 points.)
I think in the early days of Reddit, the fact that everything including comments could be voted on was intimidating to trolls, so they were kept at bay. But some time later every primal scream was rewarded, and trolls started posting in droves.
It never helped that a really low-scoring post with a high-scoring rejoinder tended to be highly ranked. That's just begging for the sort of you-got-served culture that excites trolls.