I'm often torn about offering up what I would consider constructive criticism or critiques for webcomic creators or artists. The first issues I have revolve around me. First off I realize my "criticisms" are only really personal opinions of what may or may not have worked for me. And on top of that, I (and many others I know) can come off rather preachy or overly negative, especially when offering up criticisms. And let's face it, I don't always know what I'm talking about (I'd say it's about 50/50).
Beyond that, I know a lot of creators really don't want to hear the negatives, especially from a "nobody" like me. What helps in this case is going back and looking at how the creators dealt with what I consider fair criticisms in the past. If they are met with "Well let's see you do better?" or "Your art sucks too!" or some other attack from the creator I know to just walk away, because obviously this is the so far beyond me that I can't appreciate the creators' talents and in fact, it's so good that it will never get any better. It's especially annoying though when these people ask for criticism saying they want to improve and then reacting negatively to it (ie. their call for criticism was really just a call for more people to check out their work and pat them on the back). Quick tip, if you're going so far as to ask for a critique of your work actually take the time to understand the negatives, digest them, and take a minute to really think about whether they apply or not. Never dismiss them outright.
So in the end, I hold my tongue. The odd time, if I've gotten to know the creator a bit or they've asked for some criticisms I may offer up a "maybe you could pull out every now and then to set the scene" or "you may want to avoid having the letter bubbles cover up your characters" or "didn't that guy have a beard on the last page?" but that's about it. That's probably for the best though, I haven't gotten into an internet rumble in quite some time and I like having my own little part of the web where nobody criticizes me or says anything negative. ;)