For me?
Got my TM Daytona Rossa putter on recommendation, then ran across a nugget about multiplying your power by 1.05 outside of 10 feet, and 0.95 inside of 10 ft. Still amazes me just how correct that is and how someone figured it out.
Check the reverse angle on putts. There ARE tricky little bumps and such near the hole.
Know where to land your drive depending on wind conditions and hole placement.
How about a list of one or two per of little nuggets found in these forums that greatly improved your game dramatically (maybe to your surprise)......