I'm not a huge fanfic reader, so when I do read it, it's because I want to see a logical continuation of an idea not fully explored in canon.
Yep, that's it exactly. I should add that I'm not completely anti-OC. Writers should always add an OC if they have a role that can't be properly filled by a canon character, because the canon characters should never be forced into roles that don't fit them. But I prefer that OCs be primarily shown interacting with the canon characters.