I've worked with a lot of product managers in my career. Some great, others mediocre. While working with the great ones I've tried to take note of what makes them so great. After stewing on this for a while, I finally decided to compile the traits that make those product managers so awesome.
Though not at all groundbreaking, I've found that these are the ingredients that provide for the most fertile soil from which an awesome product can sprout.
An innate sense of what the right thing is to build.
This includes, perhaps more importantly, an innate sense of what is NOT the right thing to build. The discipline to say "no, not now” and “no, not ever" is profoundly important. There will always be great ideas on things to add.
Ability to understand and control scope of a project.
A deliberate focus on the projects at hand and not getting caught up in the “but wouldn’t it be cool if it also did x” mindset.
Can reign 'em in.
Related to my first point, but a slightly different nuance. This is the ability to reign in stakeholders who all have great ideas (ideas that would extend scope, time to ship, and perhaps quality). Great ones can do this while still making sure the stakeholders feel like their voice has been heard.
Understands what a primary use case is.
Understanding of primary use cases vs. edge cases, discipline to design for the primary case, and not get caught in the trap of “what if user x does this and this and this” — while it is important to consider edge cases and make sure the system accommodates them, the purebred product manager should not allow the primary case to be disrupted and degraded to more elegantly provide for extreme edge cases.
Deep understanding of the projects in-flight, always aware of who is doing what on the project, what are the dependencies, and a full and complete understanding of the proposed solution, how it will work, and how it will look (this obviously requires close collaboration with designers). Able to give a detailed description of every aspect of the project at a moment's notice.
Persuasive and compelling.
Ability to work with developers, designers, other product managers and stake holders in a way that engenders trust and friendship. Yes, friendship. It’s amazing how much easier it is to build incredible things when everyone actually likes each other. This requires having smart answers to questions, as well as having a novice understanding of how development works, awareness of what can and can’t be built, and gut instincts on how long things take to build.
Details details details.
Focus on details. Not forgetting, neglecting, or glossing over small elements of the product that can make all the difference.
Sees the value in qualitative research. Loves to sit down and talk with the people who are using the product, understand their goals, and empathize with their struggles. The purebred product manager understands that true innovation doesn't come from data in a spreadsheet, but instead from being out and about in the real world, with real people, discovering the unseen problems.
Respectful of roles.
Maturity to understand their role vs. the role of the designer. Enough respect for the designer's role to defer to him on matters concerning how the product works and how it looks, even if he disagrees with the designer. This requires an internalization of the idea that he, as the PM, is responsible and accountable for the “what”— meaning what are we building, when are we building it, what are we building first? And the designer is responsible and accountable for the “how” — meaning how should it work, how should it look? In terms of earning trust with the designer, this is of paramount importance.
Occasionally I've even come across a product manager who lives and breathes everything I've listed. This mystical creature is the unicorn product manager. He builds products that change the world.