The allure of the show lies in the setting of a virtual reality world where one practically becomes the video game hero you have always dreamed of as a child. It probably isn't an over-generalization to say anyone who has played a number of video games as a child or has played one MMO dreams of becoming a character in such a reality. The presentation of this ego feeding world is done well enough, with the typical dungeons, towns, and fields that gamers reminisce about to how mechanics and physics work in the world. But this isn't enough to cover up the underlying pack of cliches and poor writing the show is full of.
The biggest problem with the plot is that it is expanded in to month skips. The show presents an interesting concept where the players of Sword Art Online must beat the game and if their health bar reaches zero, they are killed just like how it would be in real life. Yet, instead of presenting how the 10,000 characters adapt to this horrific news at the beginning of this nightmare, the game proceeds with the main character running off a field with a childish "I want to be the very best!" and goes on with a one month skip and a 2000 death number. And with almost each episode, another series of month skips are given without going in to full details about the main character's journey. The month skips are redundant, pass up what would be great opportunities to see the world of SAO, the struggles of characters, and just plain makes the pacing of the show out of rhythm. Flashbacks and lack of details will leave the viewer pondering many questions which can only be answered by doing your own homework: something that a show simply should not force upon viewers.
If there is one thing this show cannot be forgiven for is the characters themselves, especially Kirito. Many of the characters in the show are one-dimensional, have no real depth, and are simply awfully written. From Klein (The typical bro) to Silica (Moeblob) to Asuna (Aloof tsundere), the characters have nothing interesting to present to the viewer besides their one-defining archetype.
But the biggest offender to one-dimensional is Kiritio, a character solely created as a self-insert for a kid. From his inability to succeed in real life to becoming the envy of everyone in Sword Art Online, to episodes solely created so girls can fall for how nice he is, to his lack of struggles in the world, he is the epitome of the Gary Stu character. Typically you would expect for a main character to struggle through quests, monsters, bosses, other foul players, etc when given a show about a MMO virtual reality world. Instead Kirito proceeds to breeze through and humiliate any obstacle given. You aren't watching a character become stronger and develop, Kirito starts right off as an unbeatable character who smashes straight through everything. His only real conflict is his belief that he cannot protect his friends and comrades, a laughable messiah complex and the biggest cliche of "heroes." Kirito's character concept is nothing but childish.
With it's horrendous plot progression and even more horrendous characters, Sword Art Online can only be called mediocre at best. It presents an interesting world but suffers from a lack of exploration and finer details. The production values and soundtrack are nothing impressive and in the end one is simply left with a show that had great ideas, but was poorly executed.
