That would be misleading. There is a trace of one way incest but it's not really that big a part of the story. It's more like a side thing. (The main guy's cousin has a crush on him.) The main idea is there are video games nowadays where you can wear a helmet and 'literally' go into the game. People get trapped and so on.
There are some great reviews of SAO on Myanimelist.net which will better express my stand on SAO.
My advice: don't watch it. No offense to the fans. There are just tons of better shows that need love.
To be glib, Sword Art Online is an anime that had potential and interesting premise, but fails to utilize them properly. It's plotting is weak. The character were bleak, hollow, and poorly characterized (A good battle character would be Guts from Berserk, not Kirito). The drama is sloppily set-up. And the the second half was abysmal.
This is a very popular series. I'm not saying all popular series are bad. There are some very popular series which deserve the love and popularity, but SAO is not one of them. It is the perfect example of a series which makes up for its lack of substance with the the strength of its fanbase. It's soundtrack is pretty good though.
That question really is hard, I'd say it's the characters that ruin the plot. XD It could have been better if the characters didn't make it any worse. Eg. there are useless, oversentimental moments of oh so big "love" (when the characters didn't spend so much time together before) and whatnot.