I've been giving some thought to what it means to be a "casual" versus a "hard-core" raider. From my perspective, I think hard-core raiders are focused on the end game, specifically, progression raiding. Everything they do is geared towards that end. In order to achieve end game, they are prepared to put in long hours. Sometimes, they do things they don't particularly enjoy because it will help their end game. Sometimes, they skip things they might actually enjoy because those things won't help their raiding performance. Hard-core guilds have raiding rosters, and only the highest performers will be on those rosters. People in those guilds are competitive and that can come with an ego.
Casual players are in the game to do what is the most fun. They may also want to down the end bosses, but that will not stand in the way of doing something else that will have absolutely no bearing on raiding, just because they want to.
Note that there is nothing in that definition of casual players about how long casuals play or how committed they are to a schedule. The definition also doesn't give a benchmark for how good they have to be at playing their class or how knowledgeable they are about the game. Casuals can do or be all of those things, and more. But, the defining point for a casual player is "fun comes first". This includes life outside of WOW as much as what goes on inside the game. A casual player is more likely to drop a night of raiding for a fun event outside of WOW than a hard-core player. Within WOW, a casual player is more likely to do something goofy, just because we can and we don't have to use down time to collect things for the weekly raid.
In the end, for the casual player, it's not about the destination, it is about the journey, and how much fun can be had while on that journey. For many of us casual players, it's about the company we have on those travels. If there's a cliff we want to jump off, the company we keep is likely to start a competition to see who can launch themselves the farthest off that cliff. Or, if we see a pack of mobs along the road up ahead, well, "hold my drink and watch this". And, when it comes to raiding, we tend to ask, "Did the boss die? Did you die? Yes and yes. Great! Next!" because it's always more fun with friends.