Or they might say "that dude that rips the souls out of bad people, the one from the commercial with the black things...", referring to whatever violent first-person shooter was advertised recently on TV. I was thinking of "The Darkness" for XBox 360, but it could just as easily describe anything else.
While there are other characters I like much more, I have to agree Mario's definitely up there for being influential. Trouble is that I look at him now and wonder how someone new to videogames could see his appeal from descriptions alone (other than hearing he's been in multiple critically-acclaimed games). Guess you'd have to play to find out... once you get past the initial kiddy exterior. Most Mario games do appear to be very cheery after all.
I don't know if Pac-Man the character is influential (his later games would suggest no), but I'm so sick to death of Pac-Man that I wouldn't ever mention him in a poll like this. I lived in a time when Super Mario Bros. just came out.