Oh, it's not just poetry, my friend. I ran a photoblog for 15 years. It got a lot of views and always had a relatively high page rank, so I know folks were reading it, but comments were of two forms: either none, or "great photo". haha
People just don't know how to comment so they decide not to leave anything rather than leaving something that might sound dumb. I think all art is more a grove, you know? You feel it or you don't. It takes more introspection and thinking than most folks want to do to figure out why exactly they like it (or don't like it). So that is to say most folks have no idea why they dig something, they just do, but they feel silly commenting that alone and so they don't say anything.
That's my thoughts anyway. So I love it even when I do get simple comments such as "great photo" or "nice haiku".
Having said that, personally I always try to give a little more when I comment on other people. As geke said, 5a is my goto approach.
Yeah, I have seen that on photos as well. do comment on photos from time to time, and I try to employ a similar strategy. With photos, it's usually about a certain aspect of the photo. such as the way they light shines, the way things line up, or the colors present. A friend @dmcamera actually wrote a great article on critiquing photos. I thought that was a great. I'll see if I can get you a link.
And there's also the part where people feel more comfortable after someone else commented, but someone needs to take that first step.
Reddit somewhat bypasses it by being so large that someone will likely open the floodgates by commenting. But yeah, it's even true for non-fiction at times. For "non-art".