AngryBirds is one that immediately comes to mind. The version distributed in the Chrome Web Store is HTML5-based. Sonar is another game in the Web Store that uses HTML5.
There is nothing precluding a FPS or simulator from being implemented using Canvas and WebGL.
It can replace native games. Whether it will is for time and the gamers to decide. But it is very compelling to have a platform that is always up to date and has the most recent version of the game. No more buy a Wii or PS3 game and then wait through 20 mins of firmware updates.
Native games have the edge in 3D because WebGL is not universally supported, most notably not supported in IE. IE only recently got support for Canvas.