It depends, the tipping point is when currently undesirable behaviour gets rewarded, rather than downvoted as it currently does. In general I still feel hacker news as a community gets it right in which content and submissions are voted up and which down.
Great point. The tipping point for me on Slashdot was when insightful comments would get modded "overrated" just because they didn't agree with whatever pro/anti/MS/Linux/Apple/Democrat/Republican bias the moderators have. When the site gets so overrun by "average" users (whoever that is) with mod points that they can completely bury good comments and outright inaccurate comments get +5.
HN seems to be very good about accurate moderation. If the site ever reached the popularity of Reddit I think it would change.
However I doubt HN will follow, because unlike digg, slashdot and reddit, PG has shown no interest in gaining traffic at the expense of quality (indeed, he has called for the front page to be splashed with Erlang links, precisely so as to get those who wouldn't fit with the site to go elsewhere).