Ben Horowitz back in 1996 – Good product manager, bad product manager:
“Good product managers know the market, the product, the product line and the competition extremely well and operate from a strong basis of knowledge and confidence.
[…] Bad product managers have lots of excuses. Not enough funding, the engineering manager is an idiot, Microsoft has 10 times as many engineers working on it, I'm overworked, I don't get enough direction.
[…] Good product managers create collateral, FAQs, presentations, and white papers that can be leveraged. Bad product managers complain that they spend all day answering questions for the sales force and are swamped. Good product managers anticipate the serious product flaws and build real solutions. Bad product managers put out fires all day.
[…] Good product managers define good products that can be executed with a strong effort. Bad product managers define good products that can't be executed or let engineering build whatever they want (i.e. solve the hardest problem).”
Apparently a classic, but new to me.