Patience, age on certain beers will make you think "damn should have waited this long from the beginning"
YouTube: BobbyfromNJ (I think he's on here and if he sees this I want more videos, I loved them)
Stir plate, lost the liquor alcohol taste (due to under pitching)
Water chemistry, didn't realize how water affected brews, (the old saying "if your water tastes great, brew with it" is not true, my water tastes awesome but is so hard I can't make anything with it except a stout)
Kegging expiremental batches no longer take the extra 3 weeks of bottle conditioning I can force Carb in about 36-48hrs and instantly realize any errors. (Some flavors you learn will age out tho)