I had only considered two of that list to be absolutely essential, Metabolism (energy utilization) and Reproduction, but I see the point on the others. I also see how even these two may not be absolutely essential.

Could it be we just don't know enough yet to give a clear definition? Or is it more that the situation is so inherently fuzzy that such a definition will always elude us.

I would have thought the first was the case. OTOH, sort of continuum would seem to be a 'natural' side-effect of evolution.