@mklip2001
There is more to this game than what you'd get from just watching someone else play it. One thing you should know about it is that it has a feature that allows the player to gain completely different weapons from the bosses, depending on which of the two characters picks it up.
Basically, if Marisa picks up the weapon obtained from Cirno, she gets a spread gun whereas Alice gets nothing. If instead, Alice picks up the same weapon, she gets the snowball weapon seen in the video, and Marisa is left with nothing.
In combination with the standard Megaman type of game progression that allows players to plan their routes according to their own preferences, this allows for a great amount of freedom and strategic gameplay for the player.
There are also lots of small things that gives the game more depth. Such as the fact that Marisa gains a broom after completing four levels total, and Alice gaining the Shanghai doll (I believe) after gaining four different weapons. Then, there are also the two levels near the end where the player is forced to play only as one of the characters, and then the other, which works as a punishment for players whom have chosen to give all weapon upgrades to one character, leaving the other with nothing.
Since you were clearly unfamiliar with the game, I can't blame you for not picking up on these things from the video alone, and I'm not sure what standards of game design you're judging here, but personally, I can't see how the game design could be anything short of an A+.
Also, the music isn't good? pfft, you be trollin' :D.