I think it's more than that. The living room social gaming angle is indeed powerful. Nintendo was (still is) also smart in choosing and implement the right games for Wii's audience. WiiSports and WiiFit make day-to-day activities into a cute, relaxing and fun "vacation". The flagship titles are all rather relevant to an average person's life. No zombies, marines, mafia, aliens or intense competition is needed.
Kotaku released the most avidly played Wii games in America as of 4/1/10 and the top 3 are:
- Super Smash Brothers Brawl
- Animal Crossing
- Guitar Hero 3
Wii Fit and Wii Sports are not even in the top 10.
If you look at Sony's collection (The Shoot, Slider, Champions Online), they are mostly the same old gamer stuff. EyePet stood out because it's more like a Nintendo title in theme, but I don't know if it has the depth. A mistake is to think that casuals don't need depth. e.g., Afrika is a wonderful title, but it is rather shallow. Within a few minutes, my wife handed me the controller back and said the game is rather limited. I think Nintendo titles like WiiSport are very well made, and are able to hold consumers' attention despite Wiimote's technical limitations.
The appeal of Wii Sports is not due to a deep level of game play - it appeals because it draws in the group. I would compare it to UNO; as a family we all play this at gatherings not because it is deep but because it is fun in a family gathering among different ages.
I had lunch with a Wii gamer friend yesterday. He mentioned that he enjoys the sports games on Wii very much. But he hasn't heard of Wiimote plus. ^_^
I brought up Wii mote plus in an earlier reply and anecdotally your friends experience suports my position that peripherals that fracture the installed base are a bad idea and generally dont do well.
WiiSports alone was able to keep his family entertained and satisfied. So they won't go look for something better. However if someone show them Wiimote plus, PS Move or Natal, I'm sure they will be able to discern the differences. The challenge lies in how the vendors communicate and inspire the consumers. They need to be able to tell the differences before playing the game, even if it is a gimmicky move.
I highly doubt that the depth of gameplay is what keeps his family entertained with Wii Sports, it is more likely that they enjoy gaming together in the similar way that people still were accessing Halo or Halo 2 on Live. The social component to that interaction is what makes it fun as much as anything else once you master the game play mechanics.