Harzel
Adventurer
While I knew what THAC0 was, and I'm pretty sure I used it, and I seem to recall embracing it as an improvement over the tables, my memory is much too sketchy to remember exactly what the protocol was for players telling he what they had rolled. All that said, I would be astonished if anyone outside of a few geezers on ENWorld thought that BaB was not a vast improvement.
Of course, without a lot of research that no one would ever fund, it's not possible to be sure whether or why some people might have trouble wrapping their heads around THAC0. However, I will venture a guess as to why it has the 'reputation' of being 'complex' despite involving more or less the same arithmetic as BaB. First, I would guess that 'obscure' would be a better description of some people's negative take on it than 'complex'. Second, I would guess that this is a good example of an aspect of earlier D&D that was a barrier to entry for non-nerds. (Sure, there were lots of folks for whom THAC0 was 'no problem', the vast majority of whom I would conjecture were folks who were very facile with quantitative and abstract processes.) The source of the obscurity of THAC0 (or at least a major contributor), I think, is that the THAC0 value is defined by a hypothetical: it's the value you would need to roll if your target had AC0. Intuitively, to me at least, it is notably more difficult to understand (more obscure) how that plays into the to-hit calculation than the quantities involved in BaB - the dice roll, your to-hit bonus, and the target's AC - which are all defined in obvious ways by and related in obvious ways to the current circumstances.
Of course, without a lot of research that no one would ever fund, it's not possible to be sure whether or why some people might have trouble wrapping their heads around THAC0. However, I will venture a guess as to why it has the 'reputation' of being 'complex' despite involving more or less the same arithmetic as BaB. First, I would guess that 'obscure' would be a better description of some people's negative take on it than 'complex'. Second, I would guess that this is a good example of an aspect of earlier D&D that was a barrier to entry for non-nerds. (Sure, there were lots of folks for whom THAC0 was 'no problem', the vast majority of whom I would conjecture were folks who were very facile with quantitative and abstract processes.) The source of the obscurity of THAC0 (or at least a major contributor), I think, is that the THAC0 value is defined by a hypothetical: it's the value you would need to roll if your target had AC0. Intuitively, to me at least, it is notably more difficult to understand (more obscure) how that plays into the to-hit calculation than the quantities involved in BaB - the dice roll, your to-hit bonus, and the target's AC - which are all defined in obvious ways by and related in obvious ways to the current circumstances.