The best product managers are passionate advocates for their products and end users. They are willing to advocate for their product's users and push back against others in their company.
For example, one client of mine had the problem where the development team was given too much autonomy for developing a product. Instead of focusing on the best interests of users, they focused instead on learning new cutting-edge technology. The end result was that they padded out their individual resumes, but they created unreliable software that proved to be a customer support nightmare.
A strong product manager never would have let them get away with that. Instead, they had to bring me in after the fact to help them fix it. But, by then, the product had already suffered repetitional damage and, even after we improved the reliability, never became a best-seller.