The real problem with defining a game as Hardcore versus Casual lies not in the lack of a definition, but in the lack of information one can get from the label.
Games are not black and white. We need more than a binary definition.
I've begun to add the definitions 'fun' and 'serious' to the mix. Then we have at least 2 dimensions.
We should also add a couple of other levels to the hardcore/casual dimension, and it would do well with a third dimension as well. Then we'd actually have a valuable way of labeling games.
As it stands now, I do not put any value whatsoever to a hardcore/casual label.
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