I comment as I always have done when I see an image that appeals to me, that does not mean it is just the highest quality or skill level, simply what appeals to me at the time of viewing. I don't have the time to go systematically through the galleries so it is on a very ad hoc basis. Somedays I will go through many images and leave comments other days none at all. 
My pet hate is thumbnails that are different to the main image as I am firstly looking at the thumbnails deciding on which to view larger. I understand in the case of nudity or violence and would encourage people to post a thumbnail that can be viewed by all, but for the majority of images I would like to see the thumbnail the same as the image.
I am sure I miss many fine images that pass me by without my comment, that is unfortunate but the only practical way to participate in the site for me.  I would like to add that I am always  happy to view and pass comment on any pic if the poster requests it from me.
I do understand the reciprocal commenting and do not feel there is any harm in that whatsoever. If I walked into a bar and someone smiled and said 'good evening' I would reciprocate with a friendly greeting out politeness .

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html