I dunno much about what defines Power Metal as power metal, but most power metal has a very triumphant feel to it, like "I just slew a dragon" type stuff. X Japan songs usually feel very angry or sad, they portray a lot of very real and human emotions that you just don't see in power metal (unless you really did just slay a dragon). I've always heard them described as speed metal, but not sure what I'd call them.