In a few of your posts, you mention "Learning universally useful skills". Could you elaborate? I think I have the basic idea - public speaking, writing, negotiation, etc. - but what would you include?
Good question. You listed three in the category of interpersonal skills and yes, that's a useful category. Almost anything important you'd want to do, you'll need to work with other people to some extent. Getting better at communicating is always good. Writing, negotiating, speaking. Conflict resolution. Whatever you'd call what's described in "How to Win Friends and Influence People." Some basic understanding of sales. All useful.
Next up, I'd like to put forward the idea that numbers are either your friend or your enemy, and there is no middle ground. Being able to do basic arithmetic fast is underrated - it has a ton of value. It's easy to train up - before you get a bill or check from somewhere, try to figure out roughly in your head how much it should be. See if it matches the bill. This helps you get your mind around opportunities and costs pretty quickly and helps you check to make sure things aren't going wrong.
Probability and statistics have a ton of value in them. I learned by studying advanced baseball statistics and I enjoyed that, but there's probably other ways. Variance is super important to understand. Getting your mind around how random can be really, really random is important.
Being able to do percents, and percents of percents has a lot of value.
Basic financial accounting has a lot of value to anyone - it also helps you get your mind around numbers.
After that, execution and get-stuff-done skills are important. This could be individual stuff like building good habits, keeping a clean environment, and training yourself how to focus. Defining a scope and your goals/objectives with anything you're doing. Some of the basic formal or informal project management stuff helps.
Underrated in this category are things that you have to do a lot of - things like typing fast, knowing how to set the agenda for a meeting or phonecall so you cover everything quickly, and how to handle email without it ruining your life.
Health, obviously. Yeah, there's a ton of contradictions all the time, but it's still worth learning a little about your anatomy, the basic systems of your body, and how they wear out and fail over time (or don't, and keep working well). Nutrition. Exercise. Basic info about cardiovascular systems. Learn something about addictiveness - tolerance building, chemical addiction vs. psychological addiction, toxicity, etc.
I'd also encourage you to work towards clear thinking - rationality in the vein that Eliezer Yudkowsky writes about at LessWrong is important. It's worth spending some time to learn about how language works and how your mind works.
That's just off the top of my head - I'm sure I missed a number of important ones. Your additions in the comments, dear reader?