Rob, some months ago someone started a thread (perhaps Cardee? Sorry, I just can't remember) promoting a comment for every lo that had no comment. This thread was huge and popular. I tried to do a couple every day but eventually had to cry uncle. I have only been a member since May but the number of layouts that get posted seems to have exploded exponentially. At this point in time, I no longer post anywhere else but here but even so, it is daunting to look at everything new (posts and los) and view/read much less comment--never mind finding time to actually do some los!
My etiquette is a somewhat modified version of Kindera's. I don't comment if the lo already has a few and I have nothing new to add. I don't comment if I have nothing of substance to say (with the odd exception). I do try to comment on every lo posted in connection with any particular challenge or scrap I am doing. I do try to make my comments be specific and focussed--what really grabs me about any lo.
I like your idea of shameless self promotion

but I would hate anyone to get the idea that a lo has no comments because no one likes it. People should take some heart from views even without comments--it caught someone's eye and they took the time to look longer. I also think it is important to deposit in the karma bank--if everyone posted say 5 comments to noncomment los for every one noncomment layout that they have I dare say with over 14,000 members that that would be the end of the non-comment los.
P.S. to a long post. You can readily find non comment layouts by searching the least popular or least active. I strongly suspect that some of these people are not "active" given the dates of the layouts and the number of layouts and comments that they have made. This suspicion was the main reason I gave up on the prior thread and resolved to start at the "other end" so to speak by trying to view new layouts on a fairly regular basis and commenting there. That resolution has been severely tested by the sheer volume of layouts.