Try to explain why this conflict of interest is not in your interests, as a user of Hacker News.
Sam's post seems like it tried to anticipate this by explicitly stating that "Critical thinking is good; shallow cynicism, on the other hand, adds nothing of value to the community." They're not asking you to turn off all critical thinking about YC companies, YC startups, or the Silicon Valley investment community. They're asking that you think critically - that doesn't mean blanket dismissals, it means pointing out specific elements that are objectionable which they should change.
For example, something like: "Could you clarify whether the following types of posts might be acceptable?
1. We've had a negative experience with a YC startup that we believe the community should know about.
2. A new idea that someone posted has obvious theoretical flaws that have been encountered and documented in the literature.
3. The policy that you just wrote an essay espousing will lead to the following consequences a, b, and c, which will have these negative effects on society."
I have no connection to YC, but I'm guessing the responses to that would be "1.) e-mail the founders and try to work out your dispute with them personally fails. If that doesn't work, e-mail sama@ycombinator.com. 2.) these are fine, but link to the theoretical findings so everyone else can understand and judge them for themselves 3.) also fine."
The point is that then, your comment is specific and actionable, and readers learn something useful from it, rather than simply learning "guelo is a cynic".
Your comment looks like a lot like FUD to me. You're implying a lot of bad without even attempting to establish that anything bad has happened.
It seems to me that the way to say negative things without being accused to gratuitous negativity is to have the negative things be substantial and useful rather than just an expression of one's own cynicism.
For example, if a conflict of interest had actually reared its ugly head, that comment would be apropos. But in this case, the situation where HN used this rule to do something bad is entirely hypothetical, and is hardly an inevitable outcome.
