Saying we sample, giving examples...

Ally, I think you've pretty much hit the nail on the head there. A more precise answer might be that it's always sampling but obvious sampling is harder to pull of than non-obvious sampling. You've got the core reason why obvious sampling is harder; if the resulting song doesn't work, people will dismiss it as an uncreative rip-off.

"Stan" works largely because of the obvious sample, IMO. I can't imagine that song without Dido's hook or that particular beat. In a similar vein, "Mo Money, Mo Problems" would be completely unrecognizeable and probably not work as well without being tied to "I'm Coming Out".

True Ally, but I still think people can say there's genius sampling at work in "Stan" because by itself Dido's "Thank You" is clearly pants. If "Thank You" was musically as gripping as the use of it on "Stan", the latter's arrangement wouldn't be particularly clever.