Interesting and divisive topic! It seems I may run the risk of creating too many notifications because I have been devoting a lot of time recently to welcoming new members, which involves copy-pasting the same small variety of paragraphs. I do feel bad when I paste a welcome message on an observation that has already been IDed by several people–but I also think (and hope!) that trying to direct new users may save us all more trouble down the road.
I have occasionally commented on really good photos, but seeing the snowball effect I will try to weigh that more carefully. I do use the Favorite star, but a few times a comment seemed warranted, too. I do still think letting a new user what they have done well or could do better is a valuable interaction.