I like to leave comments a lot, because I know how happy I'll get when I recieve some for my own stuff.
At the start it was hard to figure put what else to say than 'nice', but now if I see a good dialogue or dynamics between the character, some art where you see that the creator has really put an effort or tried new angle, it's something I want to point out. It doesn't have to be super deep critique, sometimes just little effort makes someone really happy.
Thought I do comment more on not super popular works, what really makes me want to do it is when I see that the creators answer and interact with their fans.