Every once in a while I have some experience or some a point of view that I don't see reflected anywhere else. One of the benefits of the pseudo-anonymization of sites like Hacker News is that I feel a bit more comfortable stating things that don't really have a place to say anywhere else.
The only thing I regret is when I get into pointless arguments, usually when I feel that my comment was misunderstood or misinterpreted. But even those arguments sometimes force me to consider how to express myself more clearly or to challenge how deeply I hold the belief (or how well I know the subject) that lead me to the comment in the first place.