Ask HN: What is the best non-technical skill I could develop?

I'd go with effectively being able to dumb down complex concepts while correctly understanding the limitations of the project.

You don't have to be able to replicate a tech project to get a correct understanding of all its parts. Learn to communicate with software engineers in a way they find useful and act as a bridge between them and the clients you want to reach. Know the limits of things you're offering and present the product in a way that will concisely display its value.

I think any technical partner would immensely appreciate someone who has a great grasp of their work without the burden of under delivering on an ambitious sales pitch.

Selling and Marketing yourself or something you want to succeed. Its something all of us have to do, whether it be to get a new job, or convince someone to signup for your startup project. Seth Godin has a few great books in this subject area. I only mention this as I just happen to finish listening to one of his audio books last week.

It'd be worthwhile to invest in a skill you (hopefully) use for everything: decision making.

A good follow up to Managing Oneself, which it sounds like you've read, is Decisive: How to Make Better Choices in Life and Work by Chip and Dan Heath. To borrow a few direct example questions that might help you answer your own: What are some non-technical skills that would improve your next 10 minutes in life? What about the next 10 months, and subsequently the next 10 years? Imagine the tables are turned, and you are trying to provide the same advice to your best friend who happens to be in your same exact position. How would you advise them? If you could no longer do what you are currently doing, what else would you pick?

Between not knowing you, and having little information to go off, take this with a very fine grain of salt: from the strengths you outlined, especially the needing to believe in it attribute (aka aligned values) coupled with extroversion, you may be well suited for some sort of evangelism role. Maybe begin thinking about your core values and from a macro perspective what is it that you care about most. There are many organizations trying to do good and nearly all of them could benefit from a louder voice.

In your answer @lisper, you say you are not good with face-to-face? Canvassing by email borders on spamming. Effective sales is a person-to-person contact sport.

What is your track record with sales, as in revenue / sales generated?

If you can pick up the phone, reach the person you have researched, get the meeting(s) and close the deals, then you will have no problem getting good jobs. If not, then maybe that is where you should extend your skills.

Of course politics. It is key to promotion in many companies, most of the companies don't care about your technical skills or your work.