Kirito is one to Bryce Papenbrook: he was once a virtual reality gamer named Shugo Kunisaki of .hack//Legend of the Twilight who happens to have the same problem with the video game system as Kirito. Kirito's Spriggan avatar also looks like Rin Okumura of Blue Exorcist, another character voiced by Bryce Papenbrook.
"Waifu Art Online", specially the Aincrad arc (curiously, this is the only nickname that is not necessarily derogatory).
"Imoto Art Online" for the Fairy Dance arc. Also in the mix are "Harem Art Online", and "Incest Art Online".
The GGO arc (Sword Art Online II for the anime) has been jokingly referred to as "Sinon Ass Online". It's also been called Star Wars Online, on account of Kirito practically being a Jedi and Death Gun being a Sith Lord.
"Sword Fart Online" has also been thrown around.
Kirino for Leafa, due to both sharing the same voice actress, as well as being the main character's little sister.
And fans are also calling GGO Kirito as "Kiriko", of course because he looks like a girl.
Flip Flop of God: There is a little bit of this regarding whether Kirito and Sachi had romantic feelings for each other or not.
Yuuki Asuna. Konno Yuuki. Yuuki Aoi (who voiced Konno Yuuki). The Yuukis are written differently in Kanji, but even then.
Older Than They Think: Most people starting Sword Art Online will think the series was inspired by the Dot Hack series when in reality both series started around the same time in 2002. Sword Art Online was originally meant as an entry for the Dengeki Novel Prize contest but Reki Kawahara didn't submit it because it exceeded the page limit so he published it as a webnovel under the pseudonym Fumio Kunori. It was only after the popularity of his second novel series Accel World that his publishers convinced him to release Sword Art Online as an light novel in 2009.
No Export for You: Averted with most of its properties, except for the first SAO game though it has been included in the second game released overseas and the social network game.