Armchair observation from the non-professional, so take this for what it's worth: in my experience, good subtext is only half about the writing, at least when you're talking about stage and screen. A *lot* of subtext comes from the direction and acting, and doesn't necessarily exist in the words at all, save as intentional ambiguity. The best subtext is often expressed more in body language than anything else.

Even in the pure written word, good subtext is often ambiguity that only becomes clear as subtextual foreshadowing in retrospect, when a later chapter makes obvious what was going on...