First of all, let me establish: I am very biased. This creator's early work is part of what got me into Game Maker in the first place, and showed me there was something worth learning there. With that out of the way, the game.
It's hard to describe what makes this game so fun concisely, because what makes it so impressive is that it has just about everything, and functions perfectly, despite being a mostly one-button game. Rail grinding? Check. SMW-style climbable fences? Check. Walljumping? Check. Functional water? Check. Feature-complete local multiplayer? Check. You've got springs. And hooks. And launch rings. And climbable poles. And collectibles you need to grab on a time. And collectibles you need to grab in order. You've got the NSMBWii bubble feature, and the Knuckles Chaotix tether (as a completely optional feature!,) and a one-life challenge, and a level randomizer challenge. And you know what? That's not even every feature!
Despite only having a jump button, you'll be dealing with ferris wheels and pendulums (of both the platform and hurtful spiked pineapple varieties) over long drops and damaging pits in the very first stage. And somehow, it makes the whole thing completely manageable. That's my glowing review: it is an utter clusterfudgsicle of ideas and mechanics, and yet it works as smoothly as the introductory level of an official Mario game. I did not die once on the first two stages.