The concept is very cool, and something that gets you hooked right off the bat, but I really liked it for two reasons.
1. The relationship between Kirito and Asuna is beautiful, and as they fall in love with each other, I began to fall in love with them as a couple. When a writer successfully pulls this off, he just hit the _literary lottery_ (try saying those two words over and over again). I was just as happy watching them just binge together as I was watching the main plot. Happier even!
2. Because of this beautiful inner love story, the plot itself gained an added layer of depth. Suddenly I began to truly care about Kirito's journey, because I longingly wanted to see Kirito and Asuna be together again. And that makes for an great story.
