What makes a great Product Manager
Edward Ho, an Engineer at Google, have an awesome answer to a Quora Question - What makes someone a great product manager at Google? Here is an extract; modified to answer a general question - What makes a great product manager?
A Great Product Manager:
# Takes ownership of the product and all issues related to it. This goes a long way, if you own the product, you are the first one looking for bugs, first communicating with users and first worrying about whether or not you got it right. You're always first to volunteer to do the many varied tasks that go along with running a product/team like taking notes, sending emails to customers, filling things out, triaging bugs, or doing a quick mockup. Always start with: It's not someone else's responsibility, it's your responsibility.
# Be incredibly persuasive. You need to be convincing, rather than commanding to get things done. If you are already doing #1, this is easier because everyone knows you are going to be in the trenches with everyone else.
# Be an engineer. Not necessarily wiring codes but you should be curious about how the product is built as if you were an engineer. You should be interested in understanding how much something costs in engineering time, and why it costs so much. What algorithms are in use for that feature? Why is this page slow? Any engineer on the team will pay a lot of attention to big architecture decisions that affect the product and so should you.
# Be endlessly positive. Your team is likely composed of engineers, designers and some of them tend to be very cynical. A very positive Product Manager can make a world of difference in the mood of the team. You may feel silly being so positive all the time, but it's infectious and your team will feed off of it. Remember that you and your Tech Leads (lead engineers) may know of a million things to get you all down, but the rest of your team is likely not exposed to all of that. You help them do their jobs better by not wallowing in your worries because as the Product Manager, you are the team's window/messenger.
# Don't self-promote. If you attempt to self-promote, it's incredibly obvious and poisonous. Celebrate others on the team. If you are dragging around the hard work of others attempting to earn kudos for yourself, you're doing it wrong and you won't go far.
# Be inclusive. Whether it's a blog post, or a launch video for a new feature, always promote your team members.
# Fearless. The best Product Manager will speak to their higher ups the same way they speak to engineers or designers on their team. If you freeze up when questioned about your product decisions by execs, you won't be successful. Give succinct answers and be fearless when defending your team's ideas.