The overworld is a little bit simpler than Super Mario Bros 3, but more complex than Super Mario World. (No Pipes, No Flutes, No Rockes to Smash to get to Secret Areas, No items to use specifically on the overworld). The boss stages and scrolling stages are infinitely harder with more than one person. The levels are genuinely longer than what most mario games have to offer. There are on average two alternate endings per world... with each world also having two - three levels where you can go for an extra challenge (you have to carry someone through the world... not being able to use all your special powerups and such).
The only thing missing from the game is 1 - 2 more suits, and maybe a slightly more complex overworld with a few puzzles to solve, and maybe one more special house type with which to get extra items, 1ups). The level design is the best yet in a mario game (which is saying a lot). And playing with 2 players is amazing. (4 is a little chaotic but still an absolute blast if the 4 of you have a little bit of mario knowledge and are relatively calm individuals - No Psycho types allowed, you will end up with a broken remote or a broken TV or both).
