Ah yes, I suppose you're right. Maybe I should have said that we don't need a strictly defined rubric, just general guidelines. My definitions were meant more as descriptive than definitive...the point being, "casual" is just a common word, as is "serious" and "hardcore" (slang). We don't need strict parameters to know what they mean, although it doesn't hurt to have general guidelines, which I gave.
My contention is not the lack of a strict guideline, it's the lack of any guideline at all. The actual definition of "casual" is little more than a gut feeling the current speaker has, an ill-defined "them" that the speaker hopes their audience agrees with. From incident to incident the definition will change to be based on time investment, skill, mastery, achievement, money spent, or based on ephemeral levels of "caring" like you'd find among Metallica fans arguing over who's the realest fan in the group.
Because it's so rooted in emotion rather than any justifiable gradation (however loose the grades may be) the discussion is actually meaningless even though it feels true when you're caught up in it.
The only useful rubric I can suggest for "casual" that closely aligns with how it tends to be used would be one of exclusion.
Casual Player/Fan: by exclusion, those members of a player/fan base who do not identify themselves as 'hardcore,' or would not identify themselves as 'hardcore' when presented with the choice in a value-question.
This stems from my own personal observation that only the hardcore actually bother to align themselves and call themselves hardcore. If it's an issue to someone then they're probably hardcore. A non-hardcore fan would consider the whole thing to be a non-issue, and probably wouldn't ever consider it unless someone else brought it up.