One of the main skills I see to success in PMM im Empathy. Empathy in the sense of being able to to put yourself in other people's shoes. You are the customer and market advocate internallt and the product advocate externally, so understanding those different perspectives can help a LOT in any PMM materials you are developing, from slides to demos to websites to campaigns.
There is definitely overlap between brand strategy/account management and product marketing especially because the two roles operate slightly differently at every organization. Brand strategy can translate really well to product positioning, messaging and storytelling while account management can translate well to go-to-market planning and roadmaps. I answered another question above on how to break into the PMM field including a helpful storytelling exercise to build out your PMM oriented resume.
If someone is new to PMM, there are two skills I look for:
Great question. Yes, been there & struggled there before. Think ultimately the more you can make product marketing a group of strategic thinkers and enablement gurus to marketing, the more clout your team will build. If demand gen wants to parnter with PMM to understand how to market features for a campaign, if content looks to product marketing for competitive intel or market insights to build content, those are all signs you've built a PMM with real value to the marketing org.