Good lord, where do we even begin. In no particular order:

1. Non-existent story
2. No partners
3. No EXP system, rendering regular battles effectively pointless
4. All attacks are disposable, forcing you to schlep back to town after every level
5. Started the series' irritating over-reliance on the paper aesthetic

There's a lot more than that, but those are the bullets that immediately spring to mind.

Bowser doesn't talk, level-based system on a SMB3-style world map instead of cohesive interconnected environment, constant backtracking and other forms of padding, most NPCs are Toads with different colored spots instead of characters from other Mario races or brand new ones, the list goes on.