I should probably preface this with the fact that I've been a Just Dance fan for some time. That said, as far as difficulty, this game blows the Just Dance offerings out of the water. Like Just Dance and it's spin offs, there's no mat required, just one wii remote per person, and a cleared space of floor to dance on. Hold the remote in one hand (the game will show you which hand and how to hold it) and follow the movements of the characters on the screen. If Micheal has background dancers in the song, you'll have the option to choose either him or the background. The difficulty is determined by which part you follow, and which song you choose. Heal the World is pretty easy, but they mean it when they mark the difficulties on Thriller as Hard or Inhuman.
There is a warm up video you can follow with the three instructors. Playing the game and earning stars songs will unlock other training videos that break down and slow down some of the more complicated routines to help you excel. If you're looking for the routines to be exactly what some of us grew up watching on his videos, you're going to be dissapointed. The routines have been modified to play nicely with the technology available. However, they've kept it as close to the original as they could, and the result to me is quite satisfying. And exhausting!
All in all it's a fantastic game, and quite satisfying for a MJ and dance game fan like me. It takes no real skill to get into, but those who are good dancers will still have something to challenge them, and wow a party with. I still play my just dance games, but plug this one in when I feel the need to have a challenge.