It gets way harder around world 6.
and by way harder I mean that, almost exponentially harder, like I basically didn't die the first 5 worlds. And then I started dieing some, and then the bonus worlds I died like crazy.
It's difficulty design is bizzare.
not that I thought it was game of the year. I'd give it an 8.5.
I disagree with the music though, it's way way better imo than NSMB.