I tend to assume people know what things I'm referring to because so many people on the Internet RPG community use abbreviations and whatnot that I don't understand.
I'll have to agree with Nifft here.
Just play some low level, mid level and high level D&D (3e, 3.5 or 3.PF that is, because that's the kind in question) - making sure you have at least one full arcane and one full divine caster each time (preferably Cleric, in the latter case, and best of all Wizard in the former). Even if it's just one-shots or the like. Keep an eye on the "feel", so to speak, or how various high-level "facts of D&D life" go towards defining the campaign setting - and the campaign! - as it were.
I believe the original went like this: 1-5 = Gritty; 6-10 = Heroic; 11-15 = Wuxia; 16-20 = Superheroes. Or close enough. But the thing is, it's only a rough guide, and still subjective of course. See how your own definitions/interpretations go. Personally, I'd say those are not too far off - and besides, they serve well enough for the purposes of "justifying" e6 - but indeed, as always, YMMV.
All I can really tell you from experience is that the two games are very different, in "feel". In the end, I actually created a kind of (very heavily) modified e8, with slightly
less magic available than in standard e6, and that suited us even better. I doubt that would be the case for
too many groups, but maybe some (others).
I suppose the risk is, the more radically you modify a game like D&D, the more it might occur to any given DM or player, that well, you could always simply play another game altogether...
Which is why e6, though radical, works - because it's
simple. Disarmingly so. Perhaps the easiest way to tell if it's for you and yours is... try it? Assuming you've already done "the 3e thing", I guess.