Tip #1:  Merely reading a stranger's fic on the internet does not qualify you to make comments on any aspect of their life.  Unless they tell you, you do not know anything about them.  You do not know that they are stupider than you.  You do not know that they "get off" on certain genres of fiction or even things found in their own fic.  You do not know how much time they spent on said fic.  You do not even know that they are a bad writer; you can know that a particular story is bad in your opinion, but that does not make the person a bad writer.  It is possible that the fic was published a long time ago, or maybe they aren't good at writing fiction, or maybe they just don't take fanfiction that seriously.  I once knew a girl who was a great writer, but wrote horrible fanfiction (mostly because of awful Mary Sues, and bad characterizations).

Tip #2:  Don't get too bossy.  It is fine to say "I think the story would be better if you did ..." sometimes, but don't make a 50 point list and insist that the author is inconsiderate and doesn't really care about the story if they do not make all  the suggested changes within 2 hours.  Unless it is spelling or grammar problems, then your criticism will be quite subjective anyway.  An author may take your advice into consideration, may even agree with you to a certain extent, but may not heed said advice for their own reasons.  Your relationship to the author is not boss/employee.

Tip #3:  Neither is your relationship to the author parent/child.  Don't be condescending.  I personally think that it is dangerous to assume that the only reason to review is to help the author.  This is certainly one reason, but I also think that there are at least 2 others: 1.) To let other readers know how good the story is (many people read reviews to decide if they are going to read a fic) and 2.) to let the author know what you honestly thought of the finished story/chapter.  I especially think that getting it into your head that you are going to help out this inferior author can often make you sound superior and condescending. 

Tip # 4:  Give credit where credit is due.  By this I don't mean that you have to hide your criticism in mounds of praise or even use the "sandwich" method.  If you think a story needs a lot of work, I believe that you should be able to say so, but if a story has great characterization, but jumps around too much, then mention and talk about BOTH of those things.  Even talk about aspects of the story that are "okay" as opposed to horrible.  A fully fleshed out review will discuss both good and bad aspects of a story unless the fic is either mind numbingly horrible or the best thing ever.  Don't simply make a list of all of a mediocre story's flaws -- it will make you look overly critical and the author may be less likely to take you seriously than if you were fully and completely honest.

Of course I understand that when arguments get started that sometimes the fault lies with the author and many writers will go nuts, no matter how careful you are about how you leave criticism.  I still think that using these tips can do a lot to make a review more tolerable and maybe make blowups less likely.    

That is all.  If you think I have broken any of my own rules in this post, there are two possible reasons for this: 1.  I am a big, fat hypocrite.  2.  There is a difference between giving a list of general tips and leaving a review for a specific story.   Not aimed at anyone on this community, just things I've observed in reviews.  A little tl;dr, I know.