Here's how I look at it.
Forget gaming. Look at any other hobby that you could be considered hardcore in:
Wine for example. Are you a hardcore wine lover if you drink a whole heap of cheap stuff or collect a vast range of different wines?
Model trains. Are you hardcore because you play with your trains all the time or because you have a large collection and are knowledgable in all different eras and types of trains?
Film buffs. Pretty hardcore to me. They go to all the artsy film festivals, they see the movies we don't, they see the big flicks and the niche ones, they also make sure they've seen the movie back catalogue.
There are just a few examples and I don't know why people confuse 'mainstream gaming' with hardcore gaming. Actually its more like 'mainstream HD console gaming' is deemed hardcore nowadays when its just the 18 - 25 demographic gaming
