 In terms of a critique, following with "and I'm sorry to say this story did not move me from my dislike" can be useful but should only be employed when you honestly believe the author could benefit in some fashion from the comment.  (If you believe the author could benefit from no longer enjoying the pairing in question, you should perhaps seek the remedial course in "Fandom: We All Like Different Things and That's Okay.")
A review consisting entirely of how much you hate the source material on which the story is based, again unless it is followed by praise of the author's ability to make you enjoy it.  This is poor manners and it begs your ability to read a story header.  The Internet is opt-in; no one forces you to read anything here.
Calling the author names or otherwise engaging in flaming her/him based on your prior interactions with her/him.  While none of us are perfect, your reviews should be focused on the work, not the person.  If you cannot separate the two long enough to comment, don't comment.  (I've encountered this several times in my fannish history, and so I've learned not to read works by those whom I dislike too much to review without bias.)
Telling the author how you would have written or ended the story, unless it is to point out a problem with the story's structure.  (For example, a sword-and-sorcery epic that ends abruptly when the hero pulls out a pistol and shoots the villain.)  If you feel strongly that the story should have gone in a different direction, there's always room for more fanfic.  Just be sure to ask permission of the original author before you write an alternate ending to her or his 'fic.  Better still, write your own story from scratch and really show us how it's done.
Telling the author what her or his next story should be, in great detail.  Saying "I liked this and would like to see more in the same vein" is very different from saying "You need to write the prequel where this and this and this happened, and then they met this character."  One is polite and begs to be pointed to your previous work without imposing; the other is demanding and a touch childish.  A middle ground would be to contact the author privately stating that her/his work was inspiring and has given you ideas, which you would like to share, perhaps even as a co-writing opportunity, or with permission, and off-shoot fanfic universe.
A review that is entirely about your own stories and ideas instead of a comment on the story you've just read.  If you would like to discuss your story ideas with the author, that's fine.  Simply say so when you review, and give her/him the opportunity to say, "Of course I'd love to hear your ideas," or "I'm sorry, I barely have time to keep up with my own ideas right now, but perhaps we can discuss it later."