As I see it, Casual and Hardcore are both a state of mind, and depend very much on how you approach the game. I would argue that it's your play style, your approach to the game, and not your play time, that makes you Casual or Hardcore.
I like to think that a Casual player is someone who will log on and just carry on with whatever they were in the middle of doing when they logged off last. They might be grinding out another level, they might be farming materials for a profession, they might try to PuG an instance or two, or they might be raiding with their guild if they're in one. In their minds, they'll probably have a rough idea of what they want to do next – if they play World of Warcraft, they might say, "I'd like to hit level 60 before the end of the month", or "I really want to get 300 in X profession before the end of the week", an EVE Online player might say, "I'm going to try mining down in low-security space tonight", or "I'm going to play the market and see if I can get X weapon for a song", but they'll be relaxed and casual about these goals – it gives them something to aim for, but it won't be the end of the world if they don't hit them.
A Hardcore player, by contrast, logs on with very specific, well defined goals for that particular session. They'll know exactly how long they'll be logged on for, and they'll know exactly what they want to achieve in that time, and they'll have spent time theorycrafting the best way to do it. They might have set much the same goals as a Casual player – levelling their character, levelling their professions, farming materials – but they will approach the goals with a different mind set. They know what they want to do, and that's the reason they have logged on. If they don't achieve that goal, they may be upset and feel like they've failed.
To use a cheesy example – a Casual player will get more enjoyment out of the journey, while a Hardcore player will get more enjoyment out of the destination. The achievement of a goal is the reward the Hardcore player seeks/craves, while the Casual player gleans his or her reward from the process of achieving the goal.
A Hardcore player might take his or her enjoyment from the fact that their playstyle makes the game 'harder' than the Casual player – by setting themselves goals that others might find punishing, they gain bragging rights – the simple knowledge they levelled up twice or three times as fast as anyone else or that they've taken down every boss in the game. A Casual player is simply happy so long as they're enjoying whatever it is they're doing when they log on, regardless of what it is, how long they spend doing it and (within reason), regardless of their success or failure.
During the week, I probably only play WoW for a couple of hours, each night, and I consider myself to be casual. That being said, my girlfriend works all day most Saturdays, so I'll usually clean the house in the morning after she leaves, and then camp out in front of the PC for a 4 or 5 hour session, before logging off and making dinner in the evening.
Now, I think that more than a few people would say that a 4-5 hours session was Hardcore, and for some players they might be correct, but I've rarely got more than one or two goals in mind at any given time, and even then they're not set in stone. I have an idea of what I'd like to accomplish with my characters in both the short and long term, and I've got some ideas of how to get there, but I don't sweat about how I'm going to get there, or what quests I'll finish up next Wednesday after I've watched Torchwood (loving the Thomas Covenant reference in last week's episode, by the way guys - a dream come true).
While raiding the end-game instances in WoW requires that you spend large amounts of time online, is it the time you spend raiding that makes you Hardcore, or simply the fact that you're raiding end-game content make you Hardcore?
Let me put a question to you – Is it possible to be Casual and still raid in the end-game? My answer is yes, of course it is – there's absolutely nothing stopping you from grabbing 24 other people from your guild and having a stab at Tempest Keep. You may not get very far, or progress very fast, but there's nothing stopping you from giving it a go and having fun. Going in with a relaxed frame of mind, knowing that you're just there to have a look, aware that you'll wipe sooner or later can be fun – again, it all comes down to your attitude, and your approach to the game – the trick is finding 24 other people who think the same way you do, and don't have a tendency towards drama.
I know the Hardcore vs. Casual debate is an old one, probably one of the oldest, and I honestly don't see it ever being resolved until we're living in an Iain M. Banks novel and we can all create virtual copies of ourselves to live our lives vicariously. We may have to wait a while for that one though.
On a side note, check out Casually Hardcore, over at WoW Radio - Gewnora, Iolite and Gnomewise might just be my heroes when it comes to a healthy balance between MMOs and Real Life.
And there you have it – my rambling thoughts about an old, old argument. Please accept my apologies if you think I'm just rehashing an old argument, or echoing something someone else has said in the past, I'm sure you'll survive if I've stolen a few minutes of your life.