Heidegger was arguably the philosopher of the twentieth century (just as Hegel was the philosopher of the nineteenth): all subsequent philosophers (starting with Rudolph Carnap) have had the define themselves by drawing a line of demarcation, a critical distance towards him. The majority do not simply reject him; rather, they maintain an ambivalent relationship with him, acknowledging his breakthrough but claiming that he was not able to follow it to the end, since he remained stuck in some metaphysical presuppositions. For Marxists, for example, Heidegger was right, in Being and Time, to perform the turn from the exempted subject observing the world toward man as a being always-already thrown into the world, engaged in it; however, he was not able to locate human beings within the historical totality of their social practice; mutatis mutandis, the same goes for Levinas, Derrida, Rorty, some Wittgensteinians (Dreyfus), even Badiou.
Heidegger's greatest single achievement is the full elaboration of finitude, as a positive constituent of being-human--in this way, he accomplished the Kantian philosophical revolution, making it clear that finitude is the key to the transcendental dimension. A human being is always on the way toward itself, in becoming, thwarted, thrown-into a situation, primordially "passive," receptive, attuned, exposed to an overwhelming Thing; far from limiting him, this exposure is the very ground of the emergence of the universe of meaning, of the "worldliness" of man. It is only from within this finitude that entities appear to us as "intelligible," as forming part of a world, as included within a horizon of being--in short, that we take them "as" something, that they appear as something (that they appear tout court). To put it in Kantian terms: it is because of this finitude that "intellectual intuition" is impossible, that a human being can grasp things only within a gap between their mere being-there and the mode, the "as such," of their appearance; in short, that every understanding is a contingent "projecting" of a link over a gap, not a direct apprehension. The transcendental "condition of possibility" is thus the obverse of the condition of impossibility: the very impossibility for a human being to directly intuit reality, the very failure, falling-short of the goal, in what constitutes the openness of the world, of its horizon.