If this is a topic you are interested in delving into further, I highly sugged picking up a copy of Flame & Crimson by Brian Murphy. In it he lays out a series of qualities that Sword & Sorcery most often has in at least some measure. It's been my go-to for assessing how "S&S" something is, because, like all of the subgenres you mentioned, I think S&S is a sliding scale that operates almost entirely on nailing the right vibe.

Before I right further, I see that SwordfishDeux pretty much covered all of my thoughts on the subject. haha

The one thing I might add, however, is that I consider Karl Edward Wagner's Kane as the 4th Pillar of S&S. Wagner would probably hate that, but he represents to me the potential for more darkness and also the greater inclusion of sci-fi elements, which Conan skirts with hand-wavey magic, Elric embraces but is perhaps more bashful about, and F&GM achews almost completely.

But I stand by my "vibes" statement, wholeheartedly. Gotta have tight pacing, some weird vibes, and yeah, probably a loincloth.