Sword Art Online is filled with great action and Science Fiction, but I like to view it as a love story. The reason Asuna and Kazuto make such a great couple is they're both fighting to beat the game so they can be together in real life; I just think that's such an original idea for character motivation and this whole anime is based around these characters love for each other. Furthermore, you really get to see their love develop; they aren't star crossed lovers who spend all their screen time talking about how great it is to be in love, they actually cry, fight, pick themselves up, and because of this you really understand Kazuto better when he's fighting to protect Asuna.
Also, Kazuto is without a doubt the hero of this story, but he understands he isn't immortal. In some stories heroes are glorified to the point where they seem invincible, but after Kazuto loses a battle he really feels his own limitations and when he's at his lowest he actually questions whether or not he'll be able to help anyone.
This leads me into my next point which is how this anime understands the seriousness of death. "If I die in the game, I die in real life" is the slogan of this anime and that carries a lot of weight for the characters because for many characters in this anime the mere thought of dying causes them to lose their minds. The players trapped in this world aren't warriors IRL, they're regular people who now realize they have fight for their lives if they ever want to see the real word again, wouldn't you lose your mind if you were them?
Overall, Sword Art Online has a perfect balance of action and romance and I just love it!
