When it comes to critical comments, if there is a problem with the render, I usually SM that person and let them know of the problem the same goes for other than less than sterling attributes of the render.

While not an expert in lighting, it is one of the things Iearned early on and I try to keep learning as well as ehlping others with lighting issues (again, i do a lot of that via SM.) I do try to keep the OMG's and WOW's to a minimum unless the image warrants it. Most of the time though, I look at how people pose and light their renders and comment on that (as well as try to learn how they did it) So I really appreciate it when the artist goes to the effort to explain how they got that effect/pose/whatever