I'm part of a writer's group on Facebook, and people quite frequently dump a paragraph or two there and ask for criticism. In some cases it's tough to find anything useful to say, but in most cases I put together a laundry list of issues I spot. In a few cases, after pointing out their issues, I've flat rewrote it for them. I've never had someone come back and tell me they didn't appreciate my efforts. Whether their writing actually improved afterward is unknown.
Reviewing a larger piece of fiction is a bit more difficult. Generally, I'd stick to to whether or not they're following the rules, whether their descriptions are good enough, how well their actions scenes are, and how natural the dialogue comes off. I'm also from the school of thought that says fantasy dialogue should sound natural, not stilted, like it comes from 1800s England (I realize there's disagreement on this point, but like using the oxford comma and the singular their, I'm not changing my position).
The bottom line, as others have said, be kind and try to be helpful. If I had a review where they were mocking the work, I'd probably disregard the review as being written by an a-hole and ignore it. I'm way past the point where I take reviews personally. In fact, in most cases I find them more useful to other readers than they are to me.