In a lot of ways, every generation of videogame consoles several (previously unimportant) genres suddenly become pushed to the forefront of the industry and replace previously important genres; usually this shift starts to become very noticable near the end of the previous generation. In the past couple of years there has been more success in very simple (graphically unimpressive) games like Katamari and Guitar Hero; at the same time many complicated 'Mature' games (Quake 4, Doom 3) have been performing very poorly. Currently, I think the shift is a result of FPS (and 'Mature' games in general) having been "done to death" and people looking for something different; as has happened in other areas it seems that we are going through a Retro phase where people are looking for a return to simpler time ... I could be completely wrong though ...