I've come to a much better understanding of how to meaningfully define the term "casual" and "hardcore" since playing (and debating about) GW2.
To make it really simple: casual players, as I'm defining them for this blog, are players who fundamentally consider MMOGs to be experiences to be enjoyed and not games to be won. They will not do things in an MMOG that are not fun (at least not much of it) in order to pursue some kind of "win" or achievement. Wins or achievements are, to a casual player, an added bonus to their enjoyment, not something to be pursued for their own sake via otherwise unenjoyable content.
Hardcore players - again, for the purpose of this blog - are those that primarily play to win some kind of identifiable, meaningful achievement. They are there to have a top-level character in top gear and having acquired all significant achievements. Wins, top power or achievements are, to a hardcore player, the only reason to play a game. Everthing else in the game is, essentially, "fluff".
For the casual player, the fluff is what is of primary concern; for a hardcore player, the fluff is irrelevant.
Let me explain "fluff"; it is more than just fancy clothes and hairstyle and sparkly ponies in an MMOG. For instance, when a casual player can wade into a huge boss MOB fight and get to loot the shiny chest at the end, what they are getting from the chest is not important; what is important is primarily being able to be involved in such a battle and being able to loot the chest at the end.
You see, all of that is the "fluff" - even the loot, unless the loot is something that a hardcore player specifically requires to "win". The boss MOB battle is just what the hardcore player has to go through to get the "win"; it doesn't matter if the hardcore player has to grind dungeons, grind crafting, grind resources, grind map completion, jump puzzles, WvW, PvP .. all of that, to a hardcore player, is just disposable, interchangeable window dressing - fluff. What matters is the "win" - some form of power progression for their character.
To the casual player, though, even if the chest at the end just gives them a pile of stuff they're going to sell, it doesn't really matter, because they didn't do it to get the shiny "win" item at the end; they did it to experience a huge, monstrous battle and contribute enough to be able to loot the chest. They're not grinding the shiny loot at the end; they're enjoying a huge boss MOB fight for its own sake. For many casual players, such an experience has been out of our reach ever since EQ vanilla came out.
It doesn't matter how many hours you put into the game, what defines you as either a casual or a hardcore player - IMO - is what motivates you as you play. I don't see either playstyle as "wrong", but simply rather as two entirely different demographics that developers should consider in the developmental process.
GW2 has a lot of great casual-friendly features that most other MMORPGs just don't have and future MMORPGs should certainly emulate.