Here is an incisive and concise article by the wonderful Thomas Sheehan that outlines how Heidegger isn't the philosopher of Being that we have made him up to be (see my post on Zizek and Heidegger, below), with reference to the wonderful if confusing work (and its wonderful if confusing translation) of Heidegger's Beträge zur Philosophie (Contributions to Philosophy). Sheehan says, however, that instead of talking about Heidegger as a philosopher of Being, we should talk about him as the philosopher of die Sache Selbst, the thing itself or the things themselves--which he later defines as finitude. While this has the wonderful effect of basically unifying all of Heidegger's thought and connecting him to Husserl, I think it remains just as vague as it is only something that would make sense to a Heideggerian. In short, it has no meaning. While considering him as a thinker of finitude is absolutely correct, there is another way we can think of Heidegger that makes him just as unified.
For me, the best way to characterize Heidegger is as a philosopher of truth, taking his essay Vom Wesen der Wahrheit, "On the Essence of Truth," as probably the summation of his philosophical effort. I'll just briefly point out a couple more reasons why I think this is better than what Sheehan suggests.
First and foremost, mostly everything in Being and Time can be found in super-condensed form in "On the Essence of Truth." From "attunment" or mood (Stimmung) to spatiality to "historizing" and "destiny" (or, as Sheehan suggest it should be translated, "destining"), teaching this essay would allow one to extract them all and pass over most of the confusing bits. It is indicative that the reflection on truth should bring out this type of precision on Heidegger's part: that is, the first reason I think truth is Heidegger's chief philosophical occupation is in how his reflection on truth chiefly in this essay allows him to clarify much of Being and Time. Why would this be the case? Because reflecting on truth makes him adopt a standpoint that is rigidly metaphysical, almost mathematic. Being and Time begins to be seen in the light of this reflection on truth as a work that is not just a rambling and fragmentary (only 1/3 completed) meditation on the preliminary conditions necessary to reawaken the question as to the meaning of Being, but as a work unified in its inability to be completed. In other words, merely in its mode of inquiry (prompted as it is by the question as to the essence of truth) it teaches one how to read Being and Time perhaps the most important philosophical work of the twentieth-century, which is no small feat. It teaches this through situating and delimiting the term Being in its scope, so that we do not make it into a term so expansive and general that it loses its import.
Second, thinking of Heidegger as a philosopher of truth directly sets up a reading of On Time and Being, because in considering the issue of truth and determining it as unconcealment, we have to ask how that unconcealment persists or doesn't persist. Ereignis suddenly makes sense when we view Being in the light of the essence of truth.
Third, and most important, considering Heidegger as a philosopher of truth makes him seem so very close to the reflections by the the pragmatists, the Vienna Circle, and especially Wittgenstein: that is, it immediately bridges the gap between "Continental" and "Anglo-American" or "analytic" philosophy. Sheehan's definition only obscures this amazing linkage, makes Heidegger more and more into a foreign figure to the questions that have characterized this Anglo-American tradition, even as he clarifies Heidegger. That is, nothing has characterized Anglo-American philosophy more than a strict focus on truth as either graspable or non-graspable by propositional/logical statements. Even in analytic philosophy of mind the debates essentially have circled around representationalist versus anti-representationalist interpretations (think of the turn to behaviorism and then to the computational approach and now to anti-representational "phenomenological" approaches--indeed, just think over the debates over the years on qualia alone!), which reduce to the position that presentative propositional-type mental activities grasping truth or not. Heidegger as a philosopher of truth, even as early as Being and Time essentially creates this focus. I think calling Heidegger a philosopher of truth makes even this concise summary by Bertrand Russell of Wittegenstein's Tractatus even apply to him, such are the similarities:
In order that a certain sentence should assert a certain fact there must, however the language may be constructed, be something in common between the structure of the sentence and the structure of the fact. This is perhaps the most fundamental thesis of Mr. Wittgenstein's theory. That which has to be in common between the sentence and the fact cannot, so he contends, be itself in turn said in language. It can, in his phraseology, only be shown, not said, for whatever we may say will still need to have the same structure.
-Tractatus Logico-Philosophicus, Introduction, 8.
Heidegger himself says almost the same thing:
"Truth" is not a feature of correct propositions that are asserted of an "object" by a human "subject" and then "are valid" somewhere, in what sphere we know not; rather, truth is disclosure of beings through which an openness essentially unfolds/appears/is. All human comportment and bearing are exposed in its open region. Therefore man is in the manner of ek-sistence.
-"On the Essence of Truth," 127.
Even what Wittgenstein means by "show" and what Heidegger means by "disclosure" are strikingly similar, despite their different trajectories. And indeed, both defined philosophy in the light of these meditations on truth as the clarification of language with respect to truth. But what both absolutely believe is precisely the most fundamental "analytical" statement: truth, or as Russell puts it, the ability to assert with certainty, can be shown to be ungraspable by propositional statements. Calling Heidegger most fundamentallly the "philosopher of die Sache Selbst" as Sheehan suggests nowhere would emphasize the hidden philosophical companionship that analytic and continental philosophers share through this problem. And it is not as if this companionship should be stressed just because it is good in itself--I'm not arguing that. Rather, I'm arguing for this because it was the definitive and characteristic philosophical problem of the twentieth century, and remains the most pressing philosophical problem today.
Finally, to return to my first objection to Sheehan and what I said there, calling Heidegger a philosoper of truth as opposed to the philosopher of die Sache Selbst makes Heidegger intelligible in a way that everyone can understand. This is precisely because Heidegger does not have a specific sense at first for the word truth, as he does for Being and especially for die Sache Selbst. One might object and say that Heidegger has a very specific sense of truth in the end--he defines it as unconcealment, alethia. But why I put it this way (i.e. that Heidegger does not have a specific sense for truth) is that Heidegger, unlike his meditation on Being which mostly in its effort to reawaken the question of Being resignifies what we mean by "Being," engages the common understanding of truth with his specific definition. That is, what anyone means by truth, both the Heideggerian and the layman, will be addressed in Heidegger's meditations on truth because Heidegger addresses what we normally mean by truth in his specifying it as unconcealment. Thus, refering to Heidegger as the philosopher of truth, unlike refering to him as the philosopher of Being, and especially not like refering to him as the philosopher of die Sache Selbst brings one into the conversation that Heidegger is truly engaged in. In other words, it doesn't reject the rest of the metaphysical tradition and the rest of current philosophy in situating Heidegger in some special place; it does not close off Heidegger to Heideggerians only but calls forth all philosophers to this basic philosophical concept--truth--that Heidegger is engaged in revaluing and, first and foremost, just interpreting. In short, it respects the fact that Heidegger was a philosopher, and not some specialist, engaged in interpreting the rest of philosophy just like all real philosophers. Sheehan, even in his wonderful effort to clarify Heidegger, merely works to keep Heidegger more of a secret with his appellation. Reversing this tendency would be good for not only Heideggerians, but philosophy and philosophy classes in general.