When I leave comments it's because I was looking thru the Gallery and a LO caught my eye...if I view it I leave a comment, it's as simple as that. Also, I like to look at LOs by members that don't normally get a gazillion comments, they deserve just as much love b/c someone put their heart and soul into a creation and they too deserve feedback, so what if the layout doesn't look like a graphic designer put it together? Those photos, memeories, journaling and all the work that went into the LO deserve just as much attention as anyone elses.
Besides, how else would any of us evolve and learn new techniques, etc. Of course I do comment on the layouts of my "friends" too but, that is b/c their layouts continually had caught my eye and prompted me to comment and thus we became friends.
My main rule of thumb is if I view it I comment