When I really like a story and don't feel creative enough to leave a substantial comment, I often try to pick two or three (or even one!) quote from the story that moved me or that I thought was particularly cogent or that made me think, "I wish I had written that!" Often if I mention one or more of those it kick-starts my brain. If it doesn't, then I can tell myself that I love it when people appreciate a sentence or a phrase I use. (Afterall, we struggle to express ourselves so it feels great to know that someone noticed.)

If that doesn't work I might tell the writer how it made me feel--"made me laugh," "made me smile," "broke my heart." That's just me, of course. I don't have any self-consciousness about saying what I feel.

Worst comes to worst, I appreciate "Well done!" or "I really liked this story." So, something is better than nothing! Sometimes I just feel so low and yet I enjoyed the story, so, if it is on AO3, I give it a Kudo. (I hope Dawn doesn't come after me with a stick!)