DUke: Well, I think looking at preparation is just part of looking at the end goal of what you're trying to get out of the game. There's basically these scenarios:
A. Have lots of time, use it all to prepare for EVERY PTQ and PT you get so you can make money. Whether you're achieving those goals and winning, or losing, everything about this says Pro.
B. Have lots of time, don't use all of it to prepare for every PTQ and PT. A Pro wanna-be, kinda in between really (the goal's the same but just can't devote enough time).
C. Don't have lots of time but you have you spend it preparing for PTQs and PTs. Pro, because you're still spending every available moment devoted to Magic with a "money-winning" mindset.
D. Don't have lots of time and occasionally spend it on the PTQs and PT. Again, like B.
E. Have lots of time and spend every moment devoted to creating wacky decks in non-tourney or "casual" (local)tourney atmosphere. Casual. Even if it was a netdeck, since your goal is not to make money but just to play, I don't consider that serious enough (this is probably where you and YJ meet with me).
F. Have lots of time and only occasionally build wacky decks for the above goals in E. Casual.
G. Don't have lots of time but spend every moment for goals in E. Casual.
H. Don't have lots of time and only occasionally spend it building decks for goals in E. Casual.
You can see that it's mostly the goals that drive my thinking in "defining", but preparation plays a part.
Now, as I said before, a person in A can meet another person in A (say, in an airport) and play their netdecks against each other with nothing on the line. That would be a casual affair, a casual moment. But they're still not really casual players at heart. That was more like a "time-out" from their regular goals.
FoR: At first I thought Zadok's words held true, but the more I think about it, if a person in my A scenario above thinks he's a casual player despite spending every available moment going to PTQs and such, he's not really a casual player. To paraphrase, actions speak louder than "thoughts", in this case.