This is a list of some things I've learned since I registered here. Most things are from TWHL, but a few are from other sites.
1. Basic, intermediate, and 'advanced' mapping.
2. Basic modeling.
3. Gabe Newell and World Crafter share a hobby.
4. People who save lives as a job may spend every free second shooting anything that moves.
5. Trilby wearing geeks make good voice actors.
6. Luke thinks I should STFU.
7. If he can't ask politely, he should STFU.
8. Just because you've been banned from a forum 102393257358746 1/2 times, doesn't mean you have to stop going there.
9. Never pet a burning dog.
10. People with MP5s pwn people with M4s. (Actually, that's from The Rock, I suggest you watch it if you haven't)
11. Gearbox had some really bad ideas for the HD pack. Terrible ideas.
12. Kirby is a peashooter....?
13. Black Mesa will be done when it's done. AKA, 2 days before the resonance cascade. The maps will be 100% accurate.
14. Just because it seems like a good example map, doesn't mean it IS a good example map.
15.
16. There's nothing like a good MOTM review.
17. You can even recycle RMF and VMF files. Many, many times.
18. Lupus is a good diagnosis for anything.
19. If it won't do it's scripted sequence, kill it!
20. HL can have up to 16 levelchanges in a single map. This can make a FPS RPG possible for goldsource.
There's a lot more I've learned, but this is just off the top of my head.