I comment when I feel something strongly! So like, if a page is incredibly sweet and I'm touched, I'll try to remember to actually say "omg this is so sweet!!" If two characters are bickering, and their back and forth is a lot of fun, I'll try to remember to comment "oh my goodness I love this." I know as a creator, getting a ton of "HOLY CRAP WHAT" comments when something big happens feels amazing, even if those comments are small and don't leave much to reply to, so my goal in commenting is mostly so that the creator can know that I felt a thing.
So if I don't Get A Feeling, I won't usually comment. Also sometimes I just forget!
Like joanne, I don't really have fun speculating because I'd rather wait and see what's going to happen, and writing little mini-essays on the composition would feel forced from me because I don't really experience media that way -- I look for stuff like that when someone asks for a critique. My reaction isn't going to be "whoa, the colour contrast just perfectly hilights the points of interest and sets the anxious mood of the scene really well" -- my reaction is gonna be "DANG THIS IS PRETTY AS HECK, I love these colours!!"
though I am the kind of person who will page back through the comic to remember a character's name xD