Motion controls are a major factor, but not necessarily for the reasons that people think ...
Since the days of the Atari the preferred way for (pretty much) anyone to play a videogame was to play with someone else in the same room. Even a fairly mediocre game can become a shining gem when playing with 3 good friends in the same room; and one of the common arguments against the "Greatness" of the original Halo was a game like Barbie’s Horse Adventures would be a blast in a 16 player lan.
The problem Nintendo faced with the N64 and Gamecube was that, even though they had 4 controller ports, inexpensive controllers, and most households had enough people to support 3 or 4 player gaming, outside of the core gamers in a household few people would actively participate in a gaming session. Don’t get me wrong, Nintendo was doing better than most companies because you could get your girlfriend or sister to play Mario-Kart or Mario Party, but certain family members (Grandparents and Mothers in particular) showed very little interest in Gaming.
The motion controls on the Wii were specifically designed to open up gaming by making games less intimidating to non-core gamers. It wasn’t the act of flailing your arms around wildly that made the Wii successful, it was that your 60 year old mother/grandmother would play against your 4 year old nephew in Wii Sports.
Beyond that even the marketing of the Wii was to take videogames away from being the activity that a geeky-kid does in a dark basement and make it the activity where groups of people are playing together and having fun.
