Behind the Metallic Curtain

An imaginary line runs down columns and across rows in the periodic table, dividing metals from non-metals:

The diagonal line can be thought of a boundary demarcating a section of elements equidistant from Fluorine, the epicenter of electronegativity:

Roughly, the "blue" elements closest to queen Fluorine (upper right) and fanning out across the territory up to and bordering the metallic curtain are non-metals; those beyond (yellow) are deemed metallic. But what makes a metal a metal and vice versa? What is the essence of "being metallic?"