I have always been a HUGE Nintendo fan. That said, yes for me the Wii was a big disappointment. That's not to say that I regret my purchase... I'm glad I had a Wii!
I loved Galaxy and other games you aren't going to get elsewhere like Zelda, Metroid Prime and the like. I did enjoy playing through some more niche titles like Muramasa, Boy and his Blob, Xenoblade Chronicles, etc.
Now to the disappointment part. I don't have a scientific reason other then that i found myself playing my XBox 360 far more whereas my Wii turned into a party machine for when people come over (Guitar Hero, Boom Blox anyone?). I'm a believer that a good game is a good game regardless of graphics, but in last gen's case the significant difference really played a role not only graphically but how a lot of games performed. For the most part if the game wasn't made by 1st/2nd party then the quality of the game suffered greatly. Nothing felt as polished as I've come to expect from Nintendo.
Now that I have a WiiU I'm finding myself letting my 360/PS4 collect dust. The games are really excellent - I'm very happy and there's more to be excited about. !