Do not defend your work. If your work is not understood, then it either needs to be clarified or the person offering the critique wasn't reading closely enough. Let it go. If one person misreads your work, don't worry about it, but if several say the same thing, then it is worth reconsidering. Again, our readers will not always read as closely as we'd like. What is important is to understand what they are likely to miss and what parts leave the strongest impressions. Think of a scene from Don Quixote. Most of us will think of our chivalrous hero at battle with the windmill. We may have an equally memorable scene in our own works, and it won't take an expert to call it to our attention. We want to know what jumps off the page, even in a misreading...or rather, especially in a misreading. One of the best ways to find the most powerful parts of your work is to have someone skim over it and report results; then go back over it a second time for a closer reading.
Use tact in your critiques. We communicate entirely through text while online. WE don't have the benefit of vocal shifts, smiles or raised eyebrows to help communicate our thoughts, so we must choose our words very carefully. It is very easy to be understood on the Internet. One of the mot useful words in a workshop is "consider." Rather than saying, "I think you should change this part because it is confusing," you could say, "consider changing this part for clarity." The latter wording leaves the author with a better sense of control with additional options. The best critiques inspire the author to look for creative and more effective alternatives.
Do not write the work for the author. It is very tempting to inflict our own ideas on an author in our critiques. It's a natural impulse for writers, but one we should curb if we are to be helpful. So avoid comments that give specific story changes. If it is a characterization or plot problem, it is better to simply point it out and explain why you had a problem with it.
Be specific. It is of no value to simply say you like or dislike something. Saying "Awesome!" doesn't help a writer grow. We need to know why it is awesome so we can do it again. If you don't like something, try to determine why. For example, if the hero does something that you don't like, it is better to tell the author, "The hero seems to jump out of character at this point. Earlier, in the 14th paragraph, he said this and did that, then in the 30th paragraph, this happened. Is this what you intended?" Also be specific about your favorite scenes, lines and descriptions. Especially the most memorable ones.
No pain, no gain. Criticism hurts. Anybody that says different needs to have their pulses checked. Some of us can deal with it better than others, but all of us would rather hear a compliment than a criticism. Writers are generally sensitive people; we have to be in order to write. My first workshops in college seemed vicious at the time. "How could these people be so cold and heartless? Don't they know I've got feelings? I'll show them! I'll never write again!" I took everything as a personal attack in the big way. "Why can't these people see what I meant? It's plain as day to me!" Unfortunately and fortunately, everyone comes to a workshop with a different set of life experiences and cannot read our minds. Writing is learning. It is important to know how our own life experiences compare with other's. This is why we read and why we write: to find how we fit into the grand puzzle. So the pain we feel from the criticism is exactly what we need to find our differences and come closer to understanding our relationship to others, and this helps us to write so that others will understand us better.
Proofread first! Don't expect writers to proofread for you. Make sure your work is as error-free as possible before you post it. Of course, there will always be errors; most newspapers have several copy editors go over each story, and they still make mistakes. Do the best you can so readers won't be distracted from your story.