1) Be polite, for one. Like, seriously. Nothing ruins a good point worse than bad manners. People cannot accept your story criticisms, however valid they may be, if they're too butthurt over the fact you called them an idiot or whatever insulting thing you did.

2) Be brief. Because if you HAVE a good point, you shouldn't need to dedicate walls of text to it. Ever see a review for a story posted in a comment that read longer than a story chapter and took up half the screen? Ain't nobody wanting to wade through your walls and walls and walls of text in the hope of getting good advice.

3) Provide reasons for what you have to say and keep in mind the context of a story. Celestia acting like a power-mad tyrant? Well, it is OOC or alternate universe? Not adhering to canon? Well, did the fic SAY it was going to adhere to canon? Things like these.

If you want to suggest new rules or alterations to these ones, either PM or leave a comment on the main page.