In terms of raw impact on life, probably the Japanese language.<p>In terms of "clearly weaponizable in one's business/career", it's "measurement and leverage" from PG's wealth essay. <a href="http://paulgraham.com/wealth.html" rel="nofollow">http://paulgraham.com/wealth.html</a><p>In terms of "things which I wish I had known when I was much younger", I'll pick "In most things you will become the average of your five closest friends, so choose your peer group carefully." and "Dating is a sales and marketing exercise, with all that that implies." (Granted, telling myself that when I knew nothing about sales or marketing probably wouldn't have helped that much.)
"You are more valuable to your employer than they are to you".<p>That is, once you prove you're good at writing software, you can spend the rest of your career picking and choosing between employers as they suit your whim. There is very little you can do to sabotage your ability to find good gigs in this market (again, once you have established yourself as capable of doing the work).<p>That means you can treat work as a source of capital to bootstrap the rest of your life, picking it up and dropping it as need be. As you get more valuable, you can ramp down the number of months you need to work to save up the runway you need to last out the rest of the year. And you can spent the rest of your time doing whatever you want.<p>Personally, I spent a dozen years of that extra time traveling the world, climbing rocks, and building the software product business that's finally letting me skip the "employer" part entirely.<p>Start doing that when you're 25 and you will have a very fun 30s and 40s. Good luck!
That most people rarely think of you. People tend to think most of themselves, you included, me included. Not family, or loved ones of course, but the 95% of the mass of people we see on any particular day.<p>It's very easy to see one's own shabbyness of clothes or behaviour. Well, because you're right there, up close, all the time. Now, do I recall any stains on someone's pocket or thread hanging down on a dress. Maybe, but it's rare, and I have to be looking quite closely.<p>This isn't license to be smelly and gross, not bathing and wearing clothes with great greasy stain all over them. Instead it's about the +/-5% of wiggle room we get from not being under the great sharp spotlight that it feels like we're under because, well, we're ourselves.<p>All that said, it's a great idea to pay special attention to one's self for an interview. Because you'll be under great scrutiny at that point, naturally.
In terms of financial success, technical skills and know-how cannot overcome poor social skills or lack of connections, no matter how good you are technically. Outside of rare exceptions, social skills and networking are thus inescapable requirements for financial success. Money is by-and-large a social construct that has only limited correlation with quantity <i>or</i> quality of work. In my early years, I unfortunately thought that hard work alone equaled financial success.