There are several ways to write a good comment on posts. For example, you can compliment the post; mention what you specifically liked about it. That will encourage the author to continue making similar posts and it also makes them feel good about what they share with sometimes total strangers. You could also ask the questions you still have after reading. That will not only clarify your doubts, but it will also show the author that there are people out there that take an interest in what they are writing. Another way to have a high quality comment is to make connections. If you have experienced what the post is about, well, your comment can be about that, and also additional information about it.
When someone leaves a comment on my page, I expect it to be clear. I want it to be clear so that I understand what the person says, which will be easier to reply (if necessary). I would also like it to be nice. I want it to be nice because things that I share with people are things that I spent a lot of time working on, I would really like my work to be treated as it deserves to be. Finally, if someone disagree with what I said, I would like them to be honest and actually explain their opinion(s), all in a very respectful way.