Ask anyone, and they can tell you what's not good about descending AC. It's pretty simple.
Subtraction is so much harder than addition. How much of a terrible effort it must be to take one number and-
Oh, wait. It's not that hard.
In all seriousness, the ability to do short form addition inside your head is much easier then to do a subtractive operation. If you've ever been asked to do serial sevens, then you're aware of that. It clearly is a simpler operation to simply have the entire process be additive.
So what's the problem? Why not just switch over to the attack bonus? What is so great about ThAC0? They are in fact an identical operation (To hit AC 0-AC is exactly the same equation as Attack Bonus, err, mathematically if X=d20 roll and To hit AC 0 = 20-Attack bonus, then, Attack Bonus + X > AC is the same as X > (20 - Attack Bonus) - AC. tl;dr math))
Upper bounds. There is no -11 AC in Dungeons and Dragons. (Well, I'm sure in some supplement somewhere, but I'm pretty sure the intent is to have -10 AC be the absolute best armor class). This upper bound meant that the game was constrained to a human level. No matter how many bonuses you had, or how good you were, the best that you could be was -10. And there were many many difficulties in reaching such a high Armor Class. First, the best suit of armor and the best shield in the core game (AD&D1) only put you down to 2. And then to find magical enhancements on that armor of any substance was quite rare. It is possible to get both up to +5 (in what I'm sure were some high powered games) and have both a shield and armor made from "adamantite alloyed steel"
Welcome to armor class -8. Can't get any help from a ring of protection (due to wearing magical armor), so you'd have to have a bit of dexterity to push it higher. I understand it is even more limiting in the earlier versions of the game.
I'm assuming we're all familiar with what happens with the d20 system, when the values start to be larger than the dice ranges. And the fact is, with all the different kinds of stacking bonuses, I rarely have a pathfinder character that isn't trying to get his armor class into the 40's. (that's -20 AC for those of you playing along at home. That was a summoner, the party tanks were better at it. Pretty possible with armor, buffs and an enchanter in the party.
It is another thing that indicates the grounded nature of the game. You were always playing people. By 11th level, my flying, invisible, shadow walking, giant pet having, summoner often felt more than human. (However I should point out, that in contrast to 3.5, this was not outside the power level of the non-spellcasters. The barbarian was quite functional and interesting - sticky. Highly entertaining to watch him follow anyone who tried to run and smack them down. Much better then 3.5)
You could tweak and power game and min/max all you wanted. You'd hit the limit, and then the rest didn't matter. It is exceptionally hard to have a bad first edition character, no matter the stats. Discussing this with a friend, he show me how he could "build" a poorly functional fighter. It involved taking proficiencies in a lot of strange weapons. (I specialize in rope! and chair legs! oooooo. scary -_- )
It's another reason that all those little fiddly bits aren't really necessary. Descending AC, though non-intuitive and difficult for newcomers to grasp, limited the game in positive ways. It kept the focus on where it should be - the play between the players and the Dungeon Master.
I want to say thanks for all the kind comments, and hello to all my new followers. Posting has been light this last week, because I spent most of last week preparing for the traps release. Know that I'm working on an old school Alchemy & Poisons Supplement that I hope to have out next month. Also, if anyone has any ideas for these things, or questions about where you can use them, please contact me at the e-mail address underneath the picture of the monkey!