SMB is also on copious amounts of hallucinogens. The main character and his brother are plumbers. Enemies include walking toadstools, jumping turtles, a little guy in a cloud, flying squid, and whatever Bowser is. Powerups include the mushroom (which makes you grow to twice your size and effectively gives you an extra hitpoint), the flower (which lets you shoot fireballs), and the moldy (i.e. green) mushroom for an extra life, as well as the star for temporary invincibility (and melee touch attack forcing the enemy to save vs. death, or at least turn upside down and fall off the screen. I note that most Mario enemies don't do that saving throw thing very well).
The reasons for Mario's appearance and profession actually stem largely from the graphics and memory limitations of the NES cartridges. ShigeruMiyamoto? (the man largely responsible for the Mario series and a number of other projects at Nintendo) said in an interview:
"Q. What were some of the things you thought about when you created Mario? A. We had to work under technical constraints including the number of pixels and number of colors the Famicom can display. There are many reasons why we drew him the way we did. We gave him a mustache rather than a mouth because that showed up better. We gave him a hat rather than hair because that looked better, too. Mario wears overalls because that shows the movement of his arms, and he's wearing white gloves because the white contrasts better with the colored backgrounds. These are the technical reasons we made him look the way he does.
Q. So from there you decided to make him a plumber? A. We wanted him to be a character that works very hard. Also, we wanted him to be shorter than enemy characters and the princesses he rescues. Based on these factors, we decided on his appearance. We always draw the dot character first, and from that, we make other art work, for the package for example." (from http://www.miyamotoshrine.com/theman/interviews/0561.php, originally printed in NintendoPower? in some issue I no longer remember)
Fun Fact: "Alice in Wonderland" has been cited as one of the major influences for SuperMarioBrothers (think: mushrooms that make you big?).
But c'mon. You knew all of this already.