Just some background. I'm 42, in the UK, and never had a games console until 2 years ago, a gamecube for (ahem) the kids. But I got excited about the Wii after E3 coverage, preordered three so I could be sure to get one before Xmas, and sold the others at cost price to friends.
Amongst the Wii love/hate we've had, I think something is being missed. Maybe everyone takes this for granted, but I want to put it front and centre, and see if anything interesting comes out.
The wiimote is a classic of design. It's beautiful and fit for purpose. It is not perfect (e.g. not rechargeable out of the box), and the button arrangements may not be ideal. But having only got a gamecube two years ago, I can well remember the learning curve for which button is which, and even now if a thing comes on the screen saying "Hit Z" I don't always get it first time. You don't get that with a Wii, you play a tennis shot and that's that.
The thing that I think is great about the Wiimote is taking analogue control to all games (i.e. what you do is a physical analogue of the real thing). For years people have been buying special purpose analogue controllers (steering wheels, dance mats, eyetoys, bongos, guitars,..) and this takes it into every game. This really does make the gaming experience so much more visceral. You can engage in a deep way with the game so quickly if it's done right.
This does not mean that many games have got it right yet. Wii Sports unquestionably has, and I think it's a game of all time major importance (you know, like Pong, Space Invaders, ...) A lot haven't. It's more natural for some than the others.
So there you go. The Wiimote is a design classic, and Wii Sports is a game of historic importance. That's why the console is doing so well.
p.s. I use the word wiimote unashamedly. At one time I saw forum posts saying things like "It's called the Wii Remote, idiot." In the UK the word "Wiimote" is on the box.