1E didn't use THAC0. It was something introduced in the Dragonlance modules in the mid-80s IIRC, and then incorporated into 2E.3d6 said:Did AD&D1E not use THAC0? If it did, I don't think there's a substantial difference. There isn't a meaningful difference between THAC0 - 1d20 compared to AC and 1d20 + attack bonus compared to AC, except that 1d20 + attack bonus is more intuitive.
UPDATE: After looking through OSRIC, it appears that AD&D1E does use THAC0, so I don't think there's much of a difference between the AD&D system and the D&D3.x system, other than the 3.x system is more intuitive and takes up less space in the book.