> Trolls try to undermine a working system solely for their own enjoyment.

This is the most concise and accurate description of a troll on this thread (so far), thank you! As you state, this can result in undermining a working system, but more specifically, this selfishness and lack of empathy inevitably leads to:

poor communication -> negative emotions -> unconstructive conflicts

Selfish enjoyment really does get at the root of a trolls motivation, otherwise, "controversial" comments supported by sound logic, factual evidence and a genuine desire to get closer to the truth actually do add to the discussion. The real problem for a moderation system is determining if the motivation of the contributor is in line with the values of the community[1].

HN's rise in popularity and decline in quality commentary demonstrates how difficult it is to maintain the values of a community in the face of fast population growth (this just began to sound a bit too similar to arguments you hear in the immigration debate). If it's important to maintain your values, then it might also be important to mandate some level of assimilation in order to acquire certain privileges. The stackexchange sites have a fairly complex system for this and I rarely see trolls there, maybe HN should implement something similar?

[1] http://ycombinator.com/newsguidelines.html, HN's Values