Characterized (though not always) by drop-d tuning, 5/4 time, and a notable absence or minimalist approach to vocals, Post-Metal focuses on riffing and build-up to that sonic darkness that hearkens back to old Black Sabbath or Judas Priest. It's generally considered the "thinking man's" metal.
It avoids emphasizing the typical metal aesthetic: the dark, somber grimaces, the emphasis on guttural vocals that often overpower other instruments. Not to say that these ascriptions don't exist in post-metal, they're just more a byproduct than a manufactured assertion.
Post Metal seems to be about building momentum with music, getting to the immense wall of sound that eventually grinds your ass to powder. But not before some serious seduction first.
Some of the more notable bands I've encountered weave their sound beautifully. In fact, the musicianship is such that you don't notice the absence of vocals. And when they are evident, they don't dominate the production but act more as a background instrument -- a means to create a greater whole.
Perhaps most appealing to me is how conceptual the music can be, often invoking literary influences and abandoning the traditional verse/chorus/verse/chorus motif.
Top Five Post-Metal Bands At the Moment:
1) Russian Circles
**Click on the link for Pelican's "Mammoth" Video on YouTube**
5) The Ocean Collective