I've just enabled public comments on all my completed stories. I should have done it long ago, but I genuinely forgot that the option existed. I only ask that you keep it classy. If you hate a story, say so, but don't turn it into a flame-war.
Having said that, I haven't enabled comments on Trading Up and I don't know if I will once it finishes. There's been such a tide of hate from some of the people that read it that I don't want to subject the rest of you to giving them a public forum to spew more garbage. In any case, I won't enable commenting on it before it finishes and at that point, I may decide that most of the assholes have moved on and open up the comments. We'll see.