THAC0 and decreasing armor. I loved it so much. I'm not a defender of the system, I'm just a man who enjoys it.

A friend of mine once said that D&D originally was a group of differing systems working in conjunction with one another. AC and THAC0 worked well together, but they could replaced with another system, and not a whole lot needs to be changed. Same with saving throws. Same with skill systems (which is why there's two systems present in the PHB). It's better to understand "Saving throws" as one system, and attack rolls as another. Each component of the game is more or less modular in this respect--there are some lovely variable rules, however, such as turning attacks into opposed roles: Attack roll versus armor roll (d20 + AC modifiers), but twisting the game around to accept THAC0 is a bit more difficult, but the mechanics are already set. Try adding in a NEW combat system and see what you have to work with.

As for Armor Class bonuses, I never had a hard problem understanding it. Your gear's Armor Class is deducted from your base AC of 10. So a Chainmail +1 has an AC of 6. Deduct that from 10. You have your AC, 4. A ring of Protection +1 gives you an additional point of armor, 10 - (6+1) = 3. This has never been a hard concept for me to understand.

Your THAC0 is modified by their AC. You subtract their AC from your THAC0 and that tells you what you have to hit (so, To hit = THAC0 - AC). If your THAC0 is 20, and their AC is 10, you need a 10 or better to hit. In 3rd ed, with a +0 BAB and no other modifiers, you still need a 10 or better. I don't think the intent behind the mechanics have changed much. They just became less mathematically involved.

The math was an annoyance not because we weren't good at it, but because it detracted from the game. I don't think the time for combat has gone down, however, because we're doing less math, but because we're trying to remember all the variables (am I flanking this guy, did I remember to add my Bless bonuses?, et qqd.)

Still, I had fun with either. It doesn't matter to me much, but out of preference, I enjoy the d20 method more.