I would say that one of the primary differences between the two groups is how the medium is conceptualized. Core gamers tend to view video games as something greater than just a simple time waster ... whether it be a passion (that they may have a desire to turn into a career), a competitive sport (whether they participate in leagues or not), or a whole new artform (the interactive storytelling crowd).
I think it comes down to commitment. Maybe this analogy makes it simpler...
a casual gamer is to a core gamer as a kid who doodles in his notebook during class is to a professional painter
There is a casual/core crowd split in pretty much any activity, art, or field of study. As another example, I go bowling at a local lane every couple of weeks to a month or so depending on time constraints. Clearly this puts me into the "casual" crowd as I know guys that are there almost every day practicing and participating in leagues whenever they can. We are both "bowlers" but there is a clear difference in interest level and commitment. It is a passion for them, and a fun diversion for me.
