What makes a good product manager?

Pragmatism.

Yes, I'm jumping right to the point. What makes a good product manager is understanding reality.

Your product is not as good as leadership thinks it will be.

It is not as bad as sales says it is.

It is not as well-positioned in the customer's mind as marketing says it is.

It is not as difficult to support as customer support says it is.

What you have to do is be the one without the filtered glasses, and see things from a realistic perpective. Only then will you be able to prioritize your features and get your roadmap right.

Those stakeholders will milk you for everything you have to get their requests in. You need to be pragmatic. Nobody else will.