I'm in agreement with the overall consensus that criticising an anonymous exchange fic is not a cool thing to do. Even though I myself appreciate constructive criticism of my work, I realise that a lot of other writers don't want it or prefer to receive concrit privately. As long as the veil of anonymity is in place and the author's preference is unknown, I just wouldn't feel comfortable pointing out the things that didn't work for me.

However, if the anonymous author specifically asked for it in their author notes (I don't think I've ever seen this done, but I'm relatively new to the exchange scene), that would be different. Then I would probably make a conscious effort to mention something in my comments, sort of as a way of giving back to the writer. But I would keep it well balanced -- or, more likely, tilted very much towards the positive rather than the negative.