What skills are universally key to being a great product marketer?

Influencing: You need the ability to inspire and drive cross-functional teams, often without any of the "authority". Cross matrixed orgs FTW, baby.

Storytelling: You can turn a plot into a narrative (see answer above). You have a knack for finding that storytelling hook that gets to your why this solution, and why now?

Positioning: You can distill a product's features and benefits into something that's aspirational and make the audience feel like this product is uniquely suited to their needs, and can even turn them into a better version of themselves. This is true whether you're marketing an iPhone or an Enterprise Service Bus.
Curiousity: Stay hungry, stay foolish. You know you're doing a good job when you learn something new every day. 

I answered this above in my answer around soft and hard skills but if I were to pick my top, universally important skills for a PMM they would be: 

• Messaging and positioning 
• Cross-functional excellence 
• Understanding of data
• Market, customer and competitor knowledge
• Process management

From there, it really depends on your specific role and where you want to specialize. The skills each PMM has really depends on what's needed for them in their role. 

You'll often see something like go-to-market strategy/planning/execution mentioned as the most important skill for product marketers. While this isn't untrue, there is a key blend of hard/soft skills that enable a product marketer to really drive a successful GTM motion. I'll share my thoughts on the top 3 soft and hard skills that every great product marketer should have.

Soft Skills

1. Navigating Ambiguity - This is really a blend of multiple soft skills (adaptability, problem solving, organization) but is a good way to summarize THE key soft skill that every PMM should have. PMMs in particular have to deal with an exponentially higher amount of ambiguity due to the highly cross-functional nature of our role and the broad surface area that PMMs can operate in. There can also be a lack of understanding of what PMM is by other functions, especially at early stage companies where the function may be brand new. Understanding how to find opportunities to create value, structure, and alignment in the presence of ambiguity is incredibly important.
2. Empathy - Relating and understanding where someone is coming from is critical in both driving outbound initiatives and also in discerning insights from customers/users. Successful product marketing requires effective cross-functional relationships, and to build those relationships PMMs need to have a clear understanding of stakeholder objectives/goals so that alignment can be created in a way that drives success for everyone. For insights as well, we have to understand what our users/customers are experiencing in order to best help advocate for their needs.
3. Communication - Incredibly important for driving alignment, articulating ideas/plans, getting executive buy-in, speaking in front of the company, webinars, etc.

Hard Skills

1. Positioning and Messaging - Positioning is especially important if you're working in company with a complex or wide range of products, or if you're in a particularly competitive industry. Identifying how your products fit next to each other or how you differentiate against other products is critical. Once that's established, you then have to tell the story and connect with your target audience.
2. Data Analysis - There are so many different types of data sets that allow you to detect where there may be an opportunity or problem to solve, whether looking at product usage data, sales pipeline, NPS, etc. You don't need to be a data scientist, but product marketers should be comfortable rolling up their sleeves to dig through data/dashboards, understanding what type of data questions to ask, and then knowing how to partner with the relevant data experts (whether finance, data science/analytics, sales ops, etc.) to take that analysis to the next level.
3. Some sort of GTM downstream experience/expertise - Product marketers lead large cross-functional teams that ultimately handle most of the execution and it's important to understand the challenges of executing a GTM plan. Whether you've previously built integrated campaigns, launched emails, managed CRM, led SEO, worked in sales/support, etc. there's not a particular "right" answer, but the experience is valuable (and also helps provide credibility when you can really say "I get it, I've been there").

I'll take a more nuts & bolts approach to this question. The two skills I see as mandatory are writing ability and presentation ability. With the ability to write and speak well, a PMM just isn't going to be effective and is more likely to be a great product manager.

In my opinion, I think the most important skill needed to be a great product marketer is sales (specifically pitching). You have to be able to get in front of a customer and pitch your product well. This means understanding your customer's needs, explaining your value proposition in the first 90 seconds, succinctly showing how your product can solve your customer's problems, and why your product is better than other products that s/he is considering.

My ex-boss (CEO of Leadspace and former CMO of Salesforce) told me that 75% of my value as a product marketer comes from creating the perfect pitch. I know the market, I understand customer problems, I know the product, I see the competition, now I come up with a pitch that resonates with his customer base. That's why he's paying me.

If your pitch is not good, then the rest of the marketing collateral you create won't be effective. Perfect the pitch, then replicate it throughout your marketing and sales materials.

After you have your pitch down, then you can work on the other product marketing skills such as scaling your knowledge and providing strategic guidance for your business.

I explain these skills more in this blog post: https://www.leadspace.com/what-makes-great-product-marketer/

I'll touch on a few of the slightly less obvious ones:

Curiosity: I think curiosity is one of the most important traits you can have in product marketing. The role is often ambiguously defined and many times it will fall on product marketing to discover where the most effective areas to spend their time are.

Empathy: If, like me, you subscribe to the idea that product marketing is primarily a customer centric function, that means that you must be able to quickly and effectively grasp things that are important to the customer.

Internal Communication: Product marketing should be in constant communication with everyone from sales, the marketing, team, and in some cases even the CEO. If you aren't doing a good job of communicating important information to the rest of your team, how can you expect it to reach your customers?

Project Management: For lack of a better analogy, product marketers are quarterbacks. They are involved with a huge number of projects and it can be difficult to keep things straight. I'd advise all product marketers to develop a good system to keep them on track and organized. I use Asana.

Empathy is a critical skill, and it is in short supply. Your customers only care about what matters to them. Engage them in structured conversations. Listen carefully. Then reread your messaging to see if it resonates. Look to see if your message is in channels they trust. Data will tell you alot, but actual people will tell you what matters.

I completely agree with 3/4 of James' list (project management is a nice-to-have, not a need-to-have, IMO). But I'll go ahead and get even less obvious:

1. Introversion. Introverts make great product marketers because we're typically keen observers. Empathy and observational skills enable the product marketer to really understand a wide variety of audiences.

2. Writing. It doesn't help to understand a wide range of audiences if you can't communicate effectively with them. Know how to write, and how to write and what format is appropriate for a particular audience and situation.

3. Sudoku. Seriously. Product marketers spend a lot of time putting puzzles together (see: "curiosity") but Sudoku basically involves puzzles within puzzles, and a need to reconcile them with one another. That's important when messaging to audiences with different incentives and priorities.