One aspect of 2nd Edition I've never really liked is its AC/THAC0 system, and from on-line experience I know I'm not along in this opinion. I would imagine it could be very confusing to new players who've never experienced 2E before - in fact, I know it can be since the original Eye of the Beholder left me wondering why leather armor offered better protection than scale mail for quite some time. (EoB - now there's a game that needs an Enhanced Edition!)
What would be great would be if we could have an option ala IWD's 3E Sneak Attack (cheers for that BTW) to switch between the default system and that used in 3E and later editions. In other words, AC would begin at 0 and armor and bonuses would increase the number rather than decrease it. Any written reference to THAC0 would be changed to attack rolls instead; many kits, spells, items, etc already use this wording anyway. Gameplay would remain exactly the same, it would only be the presentation that changed.
For reference, here's how the AC for the games' non-magical armor would look:
Leather Armor: AC 2 Studded Leather Armor: AC 3 Hide Armor: AC 4 Chainmail Armor: AC 5 Splintmail Armor: AC 6 Plate Mail: AC 7 Full Plate Mail: AC 9