Firstly I don't see why so many people have a problem with the term "hardcore gamer", I think there is a definite need to distinguish between different niches and cultures in gaming, and at a high level the simple divide into hardcore and casual is useful.
Having said that, hardcore is an interrelated blend of playtime, taste, and mentality. I say interrelated because the more time you spend playing affects the other two. Though as I have said before you cant be a hardcore gamer and just play one game, no matter how much time you spend playing it. If you play WoW 24/7 then you are not a hardcore gamer your are a hardcore WoW addict, "gamer" is someone who plays games, plural.