This is what I think the tags should be, as well as their implied hierarchies:
There are a few advantages to tagging things like this.
First, they just look right. These are the abbreviations people already use. (Well, almost—less the ampersand, for technical reasons.) They don't look like unfamiliar jargon—they're familiar jargon. ;)
Second, they're no harder to discover than the existing scheme. People starting to tag something "dnd" will find everything except rules-cyclopedia, and if they're asking about that they're going to start typing "Rules…" anyway. Or we retag them, which is what us experienced people are for. For the same reason, all D&D tags can be added to a user's ignored or interesting tags using wildcards.
Third, people playing, say, AD&D 1st edition do not care if the tags are consistent with D&D 3.x naming conventions. In fact, they might be annoyed if they are.
Some notes on choices:
[adnd] doesn't need a "1" qualifier. 1) Almost all AD&D 1e questions are relevant to AD&D 2nd edition since it's not significantly different in most regards. 2) If people say "AD&D" without qualifying, they mean 1st edition. 3) If people mean 2nd edition specifically, they invariably say 2nd edition. Hence
[adnd] is a better tag than
[adnd1] would be, is more natural, and doesn't cover anything it shouldn't.
We don't have tags for Basic D&D (aka BX), or for the Rules Cyclopedia yet. I'm not sure we need them now, but maybe it's just because we haven't got any BD&D players here yet.
[becmi] might cover Rules Cyclopedia fine, so be might never need a tag for it. We might want to think of how to tag Basic D&D when it comes up though:
[bdnd]? I like
[bx-dnd] myself. (EDIT: Correction, we do have a
[bx-dnd] tag. I'd forgotten that I edited that into a question. In any case, I'm not wedded to that exact tag, and since I don't play BX I'm not as familiar with the preferred abbreviation.)
I think Essentials doesn't need an edition qualifier. There are almost twice the Google hits for "D&D Essentials" than there are for "D&D 4e Essentials". Both are used, but without "4e" is far more common. For our purposes,
[dnd-essentials] is just as easy to discover when typing as
[dnd-4e-essentials] would be. There's never been another edition of Essentials, and if there is such thing as "D&D 5e Essentials" at some point, we can cross that bridge with a one-time retag or synonym just fine when we come to it.
There's no "parent" tag for 3e tags. There are few uses of the 3.0 tag right now as it is, that I just don't think it matters. Everyone uses 3.5 to mean 3e generically now, only saying "3.0" if they specifically mean that edition. This is much like the AD&D/AD&D 2e situation, but the dominant, default-assumption edition is chronologically reversed.
If we really wanted to, it could be
[dnd-4] instead, since that's another common form. Either way is easily discoverable and I don't care either way