THAC0, as found in D&D, AD&D, AD&D 2nd edition and thankfully gotten ridden of in later editions of the rules.
Why do I say that? Did I hate it that much?
It's not just that THAC0 is confusing (well maybe it's a bit confusing, especially for new players), it's just not that you need to reference a bunch of character class specific tables to calculate it's values, nor is it just the fact that THAC0 involves addition and subtraction to derive, it's all that and more.
I don't think I get many readers from the old school rennaisance of gaming, most of the people I relate with online belong to the indie camp of game design so it's probably safe to criticise one of the holy symbols of early gaming (there's probably not a lot of difference between the two groups, but I'll get into that later).
Don't get me wrong, I know where the concept came from, but I've been hearing a lot of people lately who've been lamenting the demise of the old style of gaming, and just as many who have chosen to forsake the new version of D&D to return to the old styles of games they remember from years past.
Personally, I'd rather move forward than try to relive old systems that never felt right (even when they were state of the art)...