I’m pretty sure I learned more here than I did at college. But my degree is in philosophy, so take that for what it’s worth.
I can’t really think of anything right now as being the very most important thing ever, but I know if I ever have a question, and I often do, the answer is here.
I learned that in an apartment building, the ammonia smell usually attributed to cat urine can also be an indicator that your neighbor is cooking methamphetamines. Since amateur meth labs are fire and explosion hazards, I consider this a life-saving bit of knowledge.
I’ve learned all kinds of information here, but one of my methods of learning is by other peoples’ experiences. I’ve learned what a lucky person I am, based on what others here have to go through on a daily basis.
I’ve never had to deal with idiotic customers. No one at my workplace is the kind who believes National Enquirer-type stuff, or a member of the tinfoil hat brigade, or a religious fanatic. I don’t get glurge e-mail. No one has asked me if I’ve been saved, or what church I go to. My neighbors aren’t any trouble. My in-laws are some of the nicest people you’d want to meet, and we’ve never had a problem with them. My wife and I get along as well as could be expected, and we have no marital issues. Our landlords are super-nice people. We have no psychotic friends. A lot of people aren’t having the same kind of experience, and I’m profoundly grateful that I really have no complaints or problems (anymore).
Well, here was where I learned about flylady.net (yeah, I know a lot of Dopers have a problem with her, but still, this is where I learned about her), and it’s helped me and my housekeeping skills tremendously. This is also where I learned about the weight loss surgery called the duodenal switch with biliopancreatic diversion; without that knowledge, I probably would have ended up having the Roux En Wye, which, while helpful to many folks (including some Dopers), would not have been as good a surgery for me, personally.
These two things alone have made such enormous changes in my life, that it’s worth a lifetime of yearly membership fees. Everything else I’ve learned is icing on the cake.
Pineapples don’t grow on trees.
Interesting? Dunno, probably some stuff I asked about Jupiter. And the modern art threads always educate me a bit.
Useful? One of many Pit threads about overweight people that turned into a flamefest, in which I once again mouthed off like a know-it-all. It was Lord Ashtar that finally convinced me to put my words into action. 38 pounds later, I still owe him a debt of gratitude. He may well have added 20 years to my life.
Above all else, NEVER leave your PC unattended while logged into the Dope.
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I dunno about useful, but in some thread (sorry, can’t seem to find it right now) it was mentioned that the northernmost part of the continental US is in Minnesota. Ok, that’s not the interesting part (it’s most obvious looking at a globe rather than a flat projection).
The interesting part is that you cannot drive to the northernmost point of the continental US without going through Canada (well, assuming you start in the US).
Cite
Some people really believe what they say, and are prepared to go much further than I would have thought to maintain those beliefs.
Regards,
Shodan
Agreed, though in place of “people” you can substitute a number of other terms: idiots, crazies, fundamentalists, Republicans AND Democrats.
I learned that the easiest way to clean out a microwave is to boil a cup of water and then leave it in for a few moments. Wipe clean and presto! We have the cleanest microwave of anyone we know.
So thank you, to whomever passed along that cleaning tip.
The info that Hampshire posted that he got from Random in this thread just found me “under $50” that I didn’t know was out there. 
Indeed.
A corollary is that I have seen the truth of the adage that many people make up their mind first, and then look around for evidence to confirm it.
Regards,
Shodan
I know, I know - “cite?”
OK, I am pretty damn good with scientific research on Google Scholar, if I say so myself. While researching some other posters questions in GQ, I have learned a lot. Thanks, dudes.
Sadly, in my first 21 years, I had no idea (never watched enough food channel, I guess) that cooking with a blowtorch was actually something people do. I thought it was a joke until the first few posts of a thread I caught on it here recently.
I learned my new attitude about exercise - I am never going to like it, and waiting for some exercise that I like to come around is my never-ending excuse for not doing it. Just bite the bullet and treat it like a chore, and just do it.