To me, there’s nothing more important than good characterization. That doesn’t mean characters have to be good, necessarily, but they do need to elicit a strong, emotional response in the reader. There needs to be a human connection, which can only happen when you show your characters’ humanity. They need both light and darkness, flaws and fears and strengths and everything in between. They need to be complex, sometimes paradoxical, never purely black or white.
Simplicity is overrated. Perfection is boring. The very best, most memorable characters are raw and messy and complicated. Most importantly? They’re real.