What Heidegger gives us

The basic merit of Heidegger is that he develops a reading of the Greeks that is latent in all of Western philosophy, and only barely peeks out its head in Nietzsche. This is a reading that, essentially, focuses on the Idea in Plato as a development of the concept of Being, which whas developed by Parmenides. Thinking the Idea as Being means thinking against the tradition of interpretation of Plato which, since Plato, has thought of the Idea as something like the "standard," the ideal. Thinking of the Idea as a "standard" means conceiving of all things as relative to this standard, in the way that "we are all made in God's image," as is often said by the Christian scholars. The standard is something identical to itself, something that is factually existent as it is, and cannot change. This idea of the Idea as a "standard" eventually develops and reifies itself, moving farther and farther away from any notion of potential or power or activity and towards ever more refined and self-subsistent perfection in factuality through Christianity and through Descartes. Eventually it becomes something like "the formative conditions for subjective experience"--the "category" of Kant--when, as Heidegger points out, it was really never something that was definitively factual at all. Indeed, it could just have equally been considered a power (potere, in Latin--to be able).
In short, Heidegger revolutionizes (and still revolutionizes) philosophy in one fundamental thrust of his thought: interpreting the Platonic Idea not as a standard from which various "copies" derive, but as something that is what makes possible all meaning within the world. For Heidegger, the Idea is the supreme limit of the possibly meaningful, what makes the possible, to the extent that it is possible, possible. In short, it is actual (i.e. real) potential--or at least could be considered as this with as equal legitimacy as something like a "standard."