Mitchbones said:
What are the drastic differences?
From the top of my head (has been a couple of years since I stopped playing AD&D):
Instead of (Base) Attack Bonus, you have THAC0 (to hit armour class 0). ACs start at 10 and go down, and the THAC0 does as well. THAC0 of 10 means that you need to roll a 10 to hit AC0. You subtract the AC from that, so with THAC0 10 and AC 5, you hit with a 5 or better
XP Tables aren't unified - thieves advance fasters than wizards, for example
There are maximum levels for all races except humans - so race X can only advance to level Y in class Z
There are minimum ability scores for many classes. A paladin needs Cha 17, for example.
There were more alignment restrictions for classes, and on top of that, racial restrictions: Paladins had to be human, halflings couldn't be Rangers, Elves couldn't be bards.
Characters with more than one class were different: Humans had to dual-class - that's where you abandon your old class and start at 0 with a new one. Your old class abilities remain dormant until you have a higher level with your new class than with your old. You also have heavy ability score requirements for that.
All other races must multiclass - you have two or three classes at once (only some combinations allowed), and divide your XP equally. That usually meant that when your buddy was a Fighter 8, you were something like fighter 7/mage 7, or maybe 6 in both.
No PrC's. Instead there were kits - variant classes that get extra abilities from the start, but may not get certain other abilities and have extra reequirements.
No feats or skills as we know it. Instead there are weapon proficiencies - you get one every several levels (different for each class), and the penalty for using a weapon you're not proficient with differs from class to class). Not all weapons are open to learn for every class - a wizard may never learn to use the sword. If you take two points in a proficiency, you are specialized (only some classes could do that), and there's bigger specialisation, too (only for fighters, if I remember correctly).
You can also get non-weapon proficiencies instead of your weapon proficiencies. They were an all-or-nothing affair.
There were many differences in the classes. The rogue, for example, was still called thief. It had "rogue skills", which were percentile, and rolling them was absolute - if you rolled beneath your hide skill, you were hidden, no matter how good the other guy's eyes were. Instead of sneak attack, there was backstab, which was a damage multiplier instead of extra dice, and had a much narrower application.
There was no ability score formula (12/13 is +1, 14/15 +2 and so on). Instead, each ability score had its own table, which also had some benefits you only got if you had a certain class or race. The abilities went only to 25, never any higher. Plus, Strength had a special ruling: instead of 18, there was 18/XX, where XX is between 01 and (1)00. Those percentiles were only for certain characters, though.
Spontaneous spellcasting wasn't. There were no sorcerers like we know them. Clerics only had 7th-level spells and below, and 0-level didn't exist (yes, detect magic took away a slot you could otherwise use for magic missile), and only clerics got bonus spells for high ability scores. Otherwise, the ability score didn't change the difficulty of the saves the targed hat to make.
There were 5 different saving throws - I can't get them together, but they were something like "wands", "petrification/death", "other magic", and another two things. Ability scores didn't change them if I remember correctly - only class and level.