When you ask someone what type of player he or she is, you usually get the answer "hardcore" or "casual." Expanding on that, you might get "I'm a hardcore PVPer" or "I'm a casual raider." Casual and hardcore can mean so many different things to so many people that it's hard to pick just one or the other. A person who raids three nights a week might not see himself as a hardcore raider compared to the person who raids five nights a week. What about the person who doesn't raid but has an army of alts? Is he casual because he logs on and does his own thing?
I don't consider myself a casual or hardcore player. I'm the guy who has a crafting empire across his alts that I play frequently. I PVP when the mood strikes me, and while I haven't raided any of the new content yet, I run Baradin Hold every week. You can blame the alts for the lack of raiding. Some days, I might spend an hour doing my dailies or putting auctions up, and another day, I might spend 8 hours in game with a break here and there. If I had to give myself a title for the type of player I am, I would say I'm a "frequent player." A "serious player" doesn't fit right; when I think of a serious player, I think of a high-ranking arena player or one of the top raiders on his or her server.
What type of player do you consider yourself? Do you log on once a week and do a few dailies? Maybe you log in every day and do all 25 dailies before raiding for a few hours.