Sometimes I may come across a well written and essentially may have nothing really to add, I would therefore resort to a short comment describing the quality of your post or writting. I don't try to force myself to a certain standard or length of comment. It has to flow. There are things which I am knowledgeable on and there are also things which I am not.

My reason for reading other people's blogs is to learn something new. It usually has to interest me though.

The major advantage of commenting or under someone's blog post, is that it helps me to share my thoughts and knowledge on other topics which I would not write about or does not correspond to my niche.