I never understood why people hated on THAC0 so much anyway.
Because its one of those things where you, as game designer, grok it so fundamentally it doesn't occur to you that its confusing as hell to a typical customer that hasn't already spent the time grasping the concept.
Put another way, its a mechanic that emphasizes system mastery but in a very negative way; mastery isn't desired because thats how you reach a higher level of play, but because the system is ridiculously confusing and you need that mastery just to play.