QFT. I think most people don't realize how intimidating the PHB alone can be to someone whose never played before, or to a DM whose trying to step up to bat for the first time. There's alot of information, rules, nuances, etc. to remember. I think stating that 3.5 only gets complex when you add sourcebooks is understating the issue.
Yes it's based on a simple mechanic...roll d20+mod and see if you beat DC,but that's not all of it. First, there's remembering where all these modifiers come from as well as what ability determines what. There's the differences between saving throws and skill/ability checks(why doesn't my jump skill or my tumbling skill help me dodge a mechanical trap?). Then there's the "tactical" nature of combat, where rules like AoO...diagonal movement, criticals, reach, etc. must be remembered and applied. The different uses for different skills. Spells, feats, etc. etc.