Basic (what little I remember of the original boxed set I had those many years ago) was a different set of classes, like Elf and Dwarf, with Humans only being able to have mage/fighter/rogue/cleric. First and second editions were similar to one another, eventually 2nd brought in most of the classes in a similar vein to first, the magic and combat systems were pretty much the same, except instead of tables for each class/level/AC, you just worked from THAC0 which increased at the same rates as the tables for the different classes.
THAC0 is the 0 AC column from the tables of version 1, writ into the rate of change inherent to those tables. For instance fighters/rangers/paladins got better every level, thieves/assassins got a boost every other (I think), then clerics/druids got a boost every third level (I think), and mages/illusionists got a boost every 4. Thieves and Clerics might have been switched, but that's the general lay of the tables to begin with, THAC0 is just another way to say "you need a 20 to hit AC0" at first level.
It's not surprising to me that 4th has a lot of errata, because some of the stuff has created or been published with flawed combos. With the wealth of sources coming along every quarter, eventually a combo that's far overpowered can and will come along. My current pet peeve (after spending about an hour trying to figure it out) is in the new warlord builds in Martial Powers 2. I'm not sure exactly what I can trade for the Archer Warlord class feature, or even if I have to trade it in the first place for one other class feature. The other new class features are specific as to what they replace in a standard Players Handbook build, except for the Archer Warlord feature. There are two new class features for warlords in Martial Powers 1, clearly defined as replacing a PHB commanding presence, but the archer one in MP2 seems to be selectable in place of even the inspiring word feature... but why you would give up the twice per encounter healing, I don't know.
