James Reichmann explains the significance of the Thomistic distinction between esse and essence with great economy and force:
It is precisely here that Aquinas’s teaching on being differs from that of all other scholastics and from contemporary philosophers as well. Because esse is not identified with the essential determination of beings, and only because it is not, can it be viewed as common. If esse and essence were identified, the only meaning the expression, ” common being,” could have would be being as understood in the intellect, for clearly there is no essence which could be considered common to all things, unless one were to adopt a strict monist position.
This unique aspect of Aquinas’s metaphysics of being allows him to avoid the very conclusions later reached by Kant, by which metaphysics becomes identified with logic, thus rendering metaphysical inquiry equivalently superfluous.