The content of this game is mild and presented in a comic book style. As such, the violence is tame (though certain animations can look graphic - i.e., a dramatic swing of an axe) and is stylized like a comic. Additionally, the game refers to enemies and players as "defeated" rather than killed (and often times after defeating a bad guy, it is implied you merely incapacitated him to be arrested later). The writing and gameplay are definitely geared towards being more family friendly than your average game. There is very little language as well and certainly never any serious profanity. While players can create characters with (very) revealing costumes, that is the extent of any sensuality in the game (and, again, it is like a comic book).
This is, however, an /online/ game in which you interact with many other players. By and large, the community in City of Heroes is fantastic. The people are generally fairly nice and usually very helpful to new players, but as in any online game, there are less than stellar people in the community. There will be times when people will circumvent the profanity filter, or make crude jokes, or just be a jerk. This is pretty much the only reason someone would be cautious about letting their child play this game.
A very important thing to note is the strong RolePlaying community this game supports - mostly in the servers Virtue and Freedom. Mature content from the players themselves is very likely in either of those servers.
The game itself is fantastic, by the way! A great MMO. And the main review is very outdated. The game no longer requires a subscription to play!