Sometimes I don't comment because the other comments seems so well-thought-out and expertly laid out. Level of knowledge of some HN commenters is almost intimidating sometimes.
Same -- it's taken me almost two years of reading HN before actually signing up, because everything I feel like saying has normaly already been said much better than I could.
Same for me. I find that someone usually says exactly what I was thinking, but phrases it 10x better.