Written by u/Samuawesome, its a response to all the hate Sword Art Online (SAO) gets whenever there is discussion on the anime. It explains the difference between the anime and its source material and why the anime has many elements that people dislike.
This topic has been discussed to death on here. Even Reddit’s crappy search bar can find similar posts to what you’re asking.
In 2001, Reki Kawahara wrote SAO for a short story competition on the simple premise of "if players were to get trapped inside something like an MMORPG and couldn't get out, what would all those players do?" (perhaps even earlier if the prototype manga rumors are true). However, due to the word limit of the contest, he could only write a few stories rather than fully fleshing out everything and it had to be self-contained. So, SAO mainly focused on certain aspects such as Kirito and Asuna's relationship.
All the original SAO contained was basically in volume 1 of the light novels (with presumably some changes from the web novel). The novel starts with Kirito grinding on floor 74 and flashbacks to specific stories within the arc (Kayaba's hologram, the Ragout Rabbit dinner, the Kuradeel story, etc.) and then the novel finishes with the gleam eyes fight, the marriage, and the final duel.
Because the author went over the word limit, he just decided to publish SAO as a web novel instead. He then proceeded to write several side stories in the Aincrad arc (Liz and Silica's introductions, Yui's story, the moonlit black cats travesty, etc.) and moved onwards to the other arcs. By 2008, Alicization was wrapped up in the WNs.
When SAO was adapted into a light novel and then into an anime, they essentially took all that he wrote and put it into chronological order for the anime. They even asked him to write what was essentially the first arc of the progressive novels to help his original story flow better and to add more content to the anime (which they butchered lol).
One of the major gripes people have with the series is that they expected SAO to cover the 100 floors of Aincrad, but it didn’t. So, it was disappointing to them due to all the time skips and how they move on from Aincrad after 14 episodes. However, I’d argue that the SAO story as a whole never intended to stick with the death game (hence why progressive was made later on to focus on it). The Aincrad arc of the anime was just a stepping stone for the later arcs to build off of. SAO was just a simple story that later delved into how the lines between technology/virtual reality and real life were starting to get blurred. Yet, the anime was heavily marketed as one.
Another problem SAO had was that the author was a novice at writing in 2001. He used sexual assault a couple of times in his work due to him being influenced by other famous books he read. The anime adaptation plays them up a lot (especially Leafa’s scene in WoU which wasn’t even as sexual in the source material) which made people uncomfortable. However, the author hasn’t written another scene like that in the past 15 or so years now. He even constantly apologizes to the voice actors after the episodes adapting them are aired.
SAO’s anime adaptation was also riddled with a lot of problems that usual light novel adaptations have. For instance, details not being able to be adapted or rushed over, certain light novel things that can’t be adapted into a visual medium, etc. ALO (one of the most hated arcs) was especially impacted because of all the things that were cut.
The studio adapting it also makes a ton of really stupid changes here and there that source material readers have been complaining about for years. For instance, a ton of Kirisuna scenes were cut in Alicization so that the studio could add more "harem-y" scenes. The fact that a lot of anime-only people think sexual assault and goofy villains are a constant and reoccurring thing in SAO (despite sexual assault only being used twice) kinda shows how much A-1 dropped the ball with this stuff.
Kirito's characterization was also meddled with due to light novel adaptation problems and A-1's decisions. The anime adaptation does his character really dirty leading to a lot of people criticizing Kirito as "the worst MC ever" or "a typical edgy OP MC with a harem".
A-1 pictures loves playing up his cool loner side who attracts all the girls. In comparison, the Kirito in the light novels is an indecisive teenager who self doubts himself all the time in his inner monologues. He also has a genuinely witty side and he extensively observes his surroundings in new situations making it interesting to read his thought process. He has some OP moments, but he only gets them after being beaten to a pulp by villains who are way stronger than him and after he gets extensive support from his comrades. Additionally, all the trauma and character arcs he has are rushed over in the anime adaptation resulting in the viewer having to piece them together.
The last thing is that SAO is popular. A lot of YouTubers got a lot of views for their “SAO bad” videos and beat the dead horse for awhile. SAO has its flaws, but some of these videos either made things up or greatly exaggerated some of the issues rather than criticizing the actual flaws of the series. Thus, since people like parroting the opinions of the youtubers they watch instead of forming their own opinions, it results in SAO threads filled with these wrong facts being used as actual "criticisms".
Also, when it was airing, a lot of people were saying how it was "the greatest anime they've ever seen" (kind of like how Demon Slayer threads are now). People on the internet usually get annoyed by these types of statements, hence why a lot of backlash was created within the anime community.
It’s mainly a vocal minority that hate it though. More people either like it or are indifferent towards it. However, SAO being so popular meant the minority is as large as the majority of some shows.
In the end, if you liked it, then that’s all that matters.