Disclaimer: In true grognard-style I shall only consider "descending armor class" in the following. I don't care about "ascending armor class" systems, especially since the usual "selling point" of those doesn't apply once you use Delta's Target20 mechanic.
Let's begin with the classic AC 9 versus AC 10 debate: Should an average character who is not wearing any armor be AC 9 as OD&D, B/X, and BECMI profess, or AC 10 as AD&D wants us to believe? For me, any answer that doesn't immediately consult the combat tables is of a purely religious nature. It really doesn't matter whether we start at AC 9 or AC 10, what matters is whether there's a difference in running a combat. Here I prefer the B/X (and to some extent BECMI) story:
Two average, unarmored, untrained, "normal humans" should have a 50% chance per round to hit (and most likely kill) each other.
I don't know about you, but that seems perfectly reasonable to me. It certainly shouldn't be more than 50% because that would imply skill where (by definition) none should exist. And if it was less than 50% things would just drag out longer.
If you consult the B/X Basic Set, page B27, you'll see that a "normal man" indeed needs an 11+ to hit AC 9, a chance of 50% on a d20 roll. Perfect! As for the "most likely kill" part: On page B25 we find that a successful attack does 1-6 points of damage (average 3.5) while page B40 explains that "normal humans" have 1-4 hit points (average 2.5). Death incarnate!
In the BECMI Expert Set (but not the Basic Set, go figure!) we find the same "11+ to hit AC 9" for "normal man" on page 29; alas the BECMI Basic Set says that "normal humans" have 1-8 hit points (average 4.5) so things are a little less deadly (never mind the other problems this change causes, sigh).
What about OD&D? On page 19 of "Men and Magic" we find (perhaps surprisingly?) that "normal men equal 1st level fighters" which means they only need a 10+ to hit AC 9, a chance of 55% on a d20 roll. Granted, it's only a 5% difference, but that seems wrong to me. Why would an untrained combatant have the same skill as a trained one?
AD&D does away with "normal men" for the most part, replacing it with the notion of "0 level" characters (for which only humans and halflings qualify?). AD&D also recalibrates to AC 10 for "no armor" of course. Well, at least starting in the Player's Handbook it does, the Monster Manual seems to be written to the original AC 9 for "no armor" instead. But at least Gary manages to stay somewhat true to my B/X story: On page 74 of the Dungeon Master's Guide we learn that "0 level" characters need 11+ to hit AC 10, a chance of 50% on a d20 roll again. Of course average hit points are "off" as in BECMI, see page 88 of the Dungeon Master's Guide.
So then... What should "no armor" be, AC 9 or AC 10? As I said before, it doesn't matter! B/X (and to a lesser degree BECMI and even AD&D) get it "right" as far as I am concerned, only OD&D has it "wrong" since it doesn't distinguish trained from untrained combatants.
Of course there is one difference after all: I strongly prefer single-digit AC values, and so in the final analysis, AD&D is out. But I have to admit that this preference is mostly "religious" as well, not truly "technical" as it were. True, Target20 is easier with single-digit AC values, but since that's not a standard mechanic I can't really use it to "rationalize my irrationality" too much. Does "neater table layout with single digits" count?