.hack//Quantum

These two anime use the same concept involving a MMORPG game where minds of players can be trapped inside. 

Both take place mostly in a virtual reality MMO game. In both series, strange events occur such as people going into comas (in .hack), people dying in the real world if they die in the game (in SAO), and people being unable to log out (in both). I think that if you like one, you will probably like the other. 

Trapped inside a virtual world, struggling for survival, these two animes are perfect watching material for both hardcore and casual gamers. It's the future, and gaming is becoming more and more popular. Through nervegear and similar technology people are allowed to consciously live inside virtual worlds. Both animes feauture people getting trapped inside these games, unable to log out and unable to call for help. Defeat bosses, clear stages, fight hackers, these guys pretty much have to do everything just to stay alive and hope to find a way out of this virtual hellish world. 

.Hack//Quantum and Accel World are both "game based" animes. They have both a really good story and very nice character design. The animation are similar, they look both pretty good. If one is looking for a good anime, a good story and some nice animations, I would recommend anyone to check out both of these animes. 

These sci-fi anime are about massively popular (VR)MMOs and a main character meeting a very mysterious player, eventually having his/her life turned upside down as a result of meeting/becoming friends with said person. Another trait they have in common is focusing on both the game and the problems of the real world directly caused by either "The World" MMO or the Accel World.