With a game like Guitar Hero or Rock Band, it takes skill. From time spent playing it, you can progressively see yourself getting better either from hitting certain riffs or actual statistics. Wii Music does not require skill nor really have an objective. Normally, gamers would just push a title like this to the side and never think twice about it, but it represents much more coming from Nintendo. Since the core audience expected Nintendo to come into E3 announcing a new title from an established IP or possibly even a new one, people are deeply disappointed. The core, myself included, look at this as a slap in the face by Nintendo. After years of the core supporting Nintendo, their biggest announcement at what was the largest gaming convention was a pointless title directed at the casual audience and morons in general.
Quoted from GameStop's website: Musicians in your band jam by simply playing their instruments to the beat of a song or by improvising to their heart's content. Play faster. Play slower. Skip a beat, or throw in 10 more. No matter what you do, Wii Music automatically transforms your improv stylings into great music. There are no mistakes-just playing for the pure joy of playing.
That is why we hate Wii Music.