I believe the most intricate plot won't matter much to readers if they don't care about the characters, especially in a series. So I try to focus hard on making each character, whether villain or hero, have an interesting flaw that readers can relate to.
“I don't know why they're doing it. I have to assume that their motives are positive, not negative. But they don't understand the severity of the problem in this state.”
“Computers are magnificent tools for the realization of our dreams, but no machine can replace the human spark of spirit, compassion, love, and understanding.”