Firstly, its theme. We've thankfully come a long way from the days of when every MMO was a choose-your-orc adventure full of burly men with a surfeit of consonants in their names and lithe women with a deficit of fabric in their garments, but even given that, City of Heroes pushed the edges of respectibility. Superhero games can't be done, people said, but they did it. Everyone - at least anyone who ever seriously plays MMOs - has idly sketched out a superhero in the border of a school's notebook... so immediate huge audience, yeah?