For me it was the interactivity that got me at first. Then seeing everyone else enjoy it as much as I did gave me even more joy. Seeing my wife who hates video games, want to play is such a cool thing. After you get over the gimmick of motion sensing and flailing around the living room, there are other great games to play and have fun with. I got Resident Evil 4, Super Mario Galaxy and Geometry Wars for Christmas, so I will be busy for quite a while.
The next couple of games I want to get are Super Smash Bros. and Wii Fit. My wife thinks WiiFit looks awesome. Not since the original Playstation was I so excited about a console and after one year, I'm still as excited. Especially after introducing it to new and unexperienced players.
Have you tried it yet?