Sword Art Online is one of the worst animes I have ever seen.
Here are my reasons:
1) Terrible pacing. In one episode, a month has past. In about three more, two years have passed. The pacing is al over the place. The problem with the pacing is that it skips over important plot and character development.
2) Kirito is an overpowered gary stu. A gary stu is a literary term for a character who is perfect. Kirito is overpowered, has special gear that no one else has, he gets all of the girls, everyone that does not like him is evil, his fights are one-sided, and he has moral infallibility. The problem with gary stus is that they make conflicts anti-climatic because you know that they are going to win and they will win in one sided matches. At least in Naruto, Bleach, or One Piece, the main character will usually be injured in some of their major battles.
3) Cheesy romance. The romance in SAO was terrible. The romance was horribly rushed and idealized. In one episode, they kissed, has intercourse, and then got married. The biggest farce was them getting married, despite being 16 and 17 years old. They talk about how much they love each other, yet they don't even know their ages or their real names until after they were married, and we are supposed to believe this romance will last?! After ALO, the romance turned into a love triangle, revolving around a 16 year old gamer (Kirito), a comatose girl (Asuna), and his cousin (Suguha).
4) Terrible villains. The antagonists that had no sublety and were cliched; the biggest examples of this would be Kuradeel and Sugou. SAO's villain, Kayaba Ahkihiko said that he forgot the reason why he built SAO and killed thousands of people in the process. If you are going to write a villain, you better give him a good reason why he would do all the things he has done. So Kayaba is a poorly written villain, but he has the complexity of Johan Liebert compared to ALO’s villain, Sugou. Sugou is the kind of person that you find in a hentai, a rapist who is evil for no reason, only to make Kirito look like a better person. A middle aged man who wants to rape an underaged girl yet he screams like an under aged girl.
5) Not letting other characters have their moments. Klein and Aigil, the only decent male characters that weren;t wimps or villains were given little screen. It always has to be Kirito to defeat the monsters or solve all of the riddles and not letting other characters do something.
6) Bad animation. While the backgrounds look nice, when the story has to be animated, character expressions look distorted (Kirito vs Eugene is a good example of this) and action scenes look like gifs of moving light.
7) I am of Yuki Kaijura's soundtrack, but her soundtrack in SAO is lackluster. The only notable song is the coffee-soda song, which is played in almost every action scene.
8) The character consistency is bad. At one moment, Kirito will go Guts from Berserk and attack whatever he sees and then the next moment, he will act friendly for no apparent reason. Kirito is claimed to be an antisocial loner, but he has no problem talking to others and adding girls into his harem as if they were pokemon. Asuna was a calm and independent girl, who later becomes a tsundere and then becomes a damsel in distress. In fact, all the female character cannot do anything without their boyfriend Kirito.
9) The fanboys are almost as bad as the HST fanboys. They have no substance to back up their claims on why SAO is "good". Analyzing the anime objectively will result with the fanboy being raged and then try to flame you or thumb down your answer.