If you liked the .hack//Quantum anime, the Anime-Planet community thinks you'd like:
The year is 2022, and gamers have lined up on launch day for Sword Art Online, a hotly-anticipated MMORPG that lets players connect to an immersive virtual reality world with special helmets called Nerve Gear. Kirito is one such gamer who's eager to jump back into action, having spent a great deal of time as a beta tester, and quickly becomes friends with newbie warrior Klein. But soon, Sword Art Online's 10,000 players discover that not only are they unable to log out, the only way they can return to their physical bodies is by beating the 100-level tower's final boss - that is, if their HP doesn't drop to zero first, for death in the game means death in the real world. Now, with no one else to turn to, Kirito and the other participants must survive the game as best they can – some forming guilds, others ruthlessly leaving their peers behind, and many tragically falling by the wayside or to the merciless monsters populating the lands...
I believe Quantum and Sword Art Online both demonstrates a similar theme of the downside to being stuck in a game well.
While I think .hack//sign is a better recommendation with SAO for most reasons, I agree that Quantum is a good match based solely on the dark/high stakes aspect of online gameplay. Fans of one would appreciate the dark tone in the other - albeit it's much more omnipresent in SAO than Quantum.
Almost the same plot, both animes are really cool. They are about life and death struggle in MMO games (as it is written in animes decription).
In a world where everyone connects to an online network, pudgy Haruyuki Arita can't seem to catch a break. Constantly bullied, afraid to face reality and paranoid his two childhood friends pity him, the teenager uses online games as a way to escape his problems. But things change one day when Haruyuki's beautiful upperclassman Kuroyukihime approaches him with a mysterious time-decelerating program known as Brain Burst - there‘s just one catch. To keep the ability, he must defeat other Brain Burst wielders in Accel World, an exclusive, virtual MMORPG, and the newbie player will need all the help he can get. Eager to prove he's not a pathetic weakling, Haruyuki joins forces with Kuroyukihime to challenge fierce competitors and discover just how incredible Accel World and he truly are.
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.