What? 90 posts and still noone has mentioned the one thing BAB handles that THAC0 never attempted to account for in a meaningful way? One word; Multiclass. 3e was the first version that attempted to handle PCs with multiple classes in a meaningful way. Prior to 3e multiclass rules were a wierd tacked on abomination.
This is not an effect of the mechanic itself, but of the multiclass rules. You could use D20 style progression and multiclassing in AD&D as well, and THAC0 would work exactly as it does currently.
