I read mostly Naruto and Harry Potter stories. If anyone ever actually reads this and has a suggestion for a good fic for either that is not already in my favorites list, send it my way in a pm.
My reviews tend to be harsh because, unlike the majority of readers on this site, I can tell the difference between a good story and a bad one and feel no need lie or believe that fanfiction is somehow exempt from what makes a good story just because it's fanfiction and not a professionally published work. The review section is for reviews, not badly worded, misspelled, barely readable messages telling a horrible author that their horrible story is "da bes ting eva :)))))!!!!!".
There are a few things about fanfition that annoy me immensely-
1) When a bad story becomes popular/receives glowing reviews. The unfortunate reality is that this site is largely made up of people who are barely capable of reading and are thus similarly incapable of reading a story and deciding whether or not it is good. This tends to lead to badly written stories with ridiculous plots gathering 2000 favorites and reviews, the majority of which have nothing but good things to say despite the glaring plot holes and grand total of three sentences in a chapter without a grammatical error.
2) When an author feels that reviews should only say positive things. A review section is for reviews, not kiss-assing. If your story is shit, people should tell you. That is simply a risk of publishing something. If you can't take the criticism I would suggest changing your diaper and keeping anything you write to yourself.
3) When people review an entire multi-chapter story in the first chapter review section. I know most people are too stupid to understand this concept, but the chapter one review section is for reviews of the first chapter. Not the second chapter, not the seventeenth chapter, and not for the entire story. All you're doing by reviewing an entire story in that section is ruining it for the people that go there to get a feel for the story.
4) When authors add an 's' to pluralize Japanese words. If you're going to be using a different language's words, use them correctly. At least that is partially understandable considering they're writing in English, but it's even worse when they use an apostrophe. What in the world makes anyone think that the proper plural spelling of kage is kage's? Congratulations to anyone that does this, you're incorrect in two languages at once.
5) When an abandoned fic is marked complete. Complete does not mean you are done writing the story, it means that the story has reached its conclusion and that there is nothing else to add. This is the only reason a fic should be marked complete. The reason this aggravates me so much is that it actually effects how you go into a fic. If you know, or in this case think you know, that a story is finished you go into expecting the ending to occur at that particular point and this changes how you look at events in the story. What was once just a normal, unimportant story piece is now made significant because the ending has been marked as x number of chapters away. It is also beyond jolting to get to a final chapter expecting a conclusion and to instead read some excuse for why the author has given up.
6) When an author puts anything, absolutely anything, in the summary of a story which is a sequel to another story besides "Sequel to such and such". It's very simple, if someone hasn't read the first one, they aren't going to read the second, and if someone has read the first they're either going to read the second or not, all putting that summary right there is doing is spoiling the story for anyone that finds the sequel before the first part. If a summary is necessary, put it in the story itself where it isn't going to ruin the story for people.
7) When stories are edited because of the rule against MA material. My stance is very simple on this: the rule is ridiculous and any author that modifies their work to follow it is pathetic.
8) When authors use the words sandaime, yondaime, etc. as titles by themselves. They are not titles. Hokage is a title, sandaime is simply specifying which Hokage, Kazekage, etc. is being talked about. Sandaime itself simply means third, not The Third so it makes absolutely no sense to say something along the lines of "Sandaime picked up his pipe and took a puff." It'd be a bit like saying "Fourty-fifth signed the new bill into law." They're not perfect matches, but the principle stands. If you're going to use another language's words in your writing, have the courtesy to use them correctly.
9) When authors bash. I get it, we're a community who spends its time reading and writing fanfiction, it's inevitable that some of our members will be pathetic enough to spend their times writing stories to insult those characters they don't like. And some of those people will even be pathetic enough to pretend those characters have absolutely no reason to be the way they are *cough* Sasuke *cough*. Or that some characters aren't the product of the faulty systems employed to train or raise them *cough* Sakura *cough*. Or that not even the great are perfect, but they do the best they can *cough* Dumbledore *cough*. But it's one of the absolute quickest ways to ruin a story. No story, absolutely none, can survive bashing and come out of it good. There's no reason to ignore a character's faults, but it's ridiculous to bash.