OpsKT
Explorer
For me it was more about the fact that the math wasn't consistent for all the different things you did. You subtract with THAC0, add with damage, roll under d100 for thief skills, roll over d20 for saving throws, gain % bonuses to XP based on Prime Scores, and your AC goes down as it gets better. None of those is hard or individually problematic, but taken all together I think you can see where the system is inelegant and somewhat confusing.
For me I always referred to AD&D as, "mathematically schizophrenic."