Other than overall mood I fail to see the difference.
Mario jumps on platforms and shoots projectiles. Mega Man jumps on platforms and shoots projectiles. In Kirby and Crash Bandicoot you can attack with melee attacks. In Ninja Gaiden and Castlevania you can attack with melee attacks.
Really?
Pure platformers are focused on collecting things, level exploration, and simply just beating levels. There are enemies and boss battles, but that's not the focus.
Action-platformers focus on combat, boss battles, and survival in addition to the platforming.
You get a distinctly different gameplay feel from Mario than you do Mega Man.