For me, casual vs. hardcore (in regards to the type of gamer) simply means how much an individual is into something. A casual gamer, for example, may buy only a couple of games a year and average only a few hours a month for gaming, while a hardcore gamer may buy several games a year, and dedicate at least a few hours a week to gaming. I think people are needlessly adding too much emotion to these terms (e.g., cool vs. uncool, mature vs. immature, etc.), when it's simply a measure of how much someone values gaming, a genre, or so on.
Then of course there are games aimed at casual play (i.e., designed for quick play sessions) which isn't exactly synonymous with a casual gamer, because someone who plays a lot of video games may still enjoy those types of games. Just as there are hardcore Scribblenauts fans who dedicate hours each day to a game, there are casual gamers who play Call of Duty and only average a small amount of hours across the year (likely front-loaded with release).