Thank you for this! Here are a couple skills, one each under "Soft" and "Formal" that I have found to be very helpful in developing your career and craft.

Soft Skills

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Presentation - Sometimes how you present and talk about your solution becomes more important than the solution itself. And this is where I find a lot of UX designers not doing enough justice to their talent. Whether it's presenting to clients, management or peers in design and development teams, they must develop the skills to engage their audience and absorb feedback without getting defensive.

Formal:

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Motion Design - Just like in the case of Visual Design, it's very useful to develop baseline skills in motion design so designers can use motion to guide various stakeholders on how the product interactions will actually behave for the users in real life. The devil is in the detail and that's what get lost in "static" prototypes that are unable to demonstrate all the micro-interactions necessary to bring your idea to life.