These days, I raid three nights a week, three hours a night or so. To me, it's far, far more casual than what I used to do. I don't spend a ton of time on farming materials, and I don't spend a ton of time on other things unless it happens to be in game holiday time. I don't usually run random instances unless I'm after something specific, and I don't really do PvP at all. I'd call myself casual, simply based on the time that I play and what I do with that time. Yet there are still plenty of people out there who fling the hardcore title at me.
When I look at how I used to play and how I play now, I can't understand why anyone would think what I'm doing now is anything but casual, and I'm confused as to why anyone would say differently.
It's a really good question, and I don't know if there's really a defined answer to it because the question itself is so subjective. To someone like me who spent endless hours plugging away in Molten Core and trying to smush Ragnaros into a fine paste, what I do now is much less hardcore, far more casual, and just my speed. But to someone who's never stepped a foot into a raid instance in their gaming lifetime, even the little raiding I do must look like a lot.
To our realm, maybe we look hardcore. To the rest of the world, I expect we look somewhat casual. It's all a matter of which perspective you're looking at the situation from, I think. As far as I am concerned, my guild and my playstyle is entrenched deeply in that somewhere in between, and I'm perfectly OK with that. It's exactly where I want to be.
I think what bothers me more than the definitions is the way that the terms casual and hardcore are thrown about like insults, to be perfectly honest. Having played both ends of the spectrum, I can firmly say that there's nothing really wrong with either, but I think that most players out there are somewhere in that in-between space. What I think is most interesting, however, is that WoW seems to be steadily moving, developmentally speaking, to a space that is neither casual nor hardcore. It's somewhere in between -- somewhere where all of us already are.