The most defining characteristic of a massively multiplayer online game is the very fact that it's massively multiplayer. Until the advent of the MMOG (and yes, I'm including MUDs as MMOGs) playing video games was either a solitary experience or one you experienced with a few existing friends. MMOGs are wonderful in that they allow us to meet new people across the globe; breaking the physical boundaries of our real-world environment that would otherwise prevent our interaction. The relationships formed and fostered during our time gaming often end up transcending the game itself.
Take my friend Rob, for example (Please! Ha!) Rob and I have been friends since middle school and went to college together in Florida. After college Rob got married and eventually moved to Atlanta, Georgia. MMOGs have been great for us because they've allowed us to stay in touch while enjoying a hobby we both love. When we started to play Dark Age of Camelot, Rob met a guy online named Josh who lives in California. Josh played a tank class and Rob played a healing class. Since both of these guys are arrogant goofballs with a similar sense of humor they hit it off right away. As they played together, learning their classes as they went, they became excellent players of their respective classes. Over the years they've stuck together through Star Wars Galaxies, City of Heroes, Everquest 2, Lord of the Rings Online and, of course, World of Warcraft. Although they do mix it up occasionally they almost always stick to the same roles of healer and tank. As a result of knowing their roles, and knowing each other so well, they typically dominate whatever game they play; whether it's in PvE or PvP. But more than just becoming great gamers, they've become great friends.