I'll start with the first thing I wish I knew.
It was probably about 18 years ago when a buddy and I went fishing with his dad and he let me try his rod and reel. I had never fished with a baitcaster and just let 'er rip! Had no idea I had to feather the spool and got the worst bird's nest ever. He was pissed.
Flash forward to 5-6 years ago I tried a baitcaster again. Did not know you needed to adjust the reel with each different lure you tied on. After many failed backyard practice sessions I gave up on learning how to cast with it and sold my baitcasting reel.
Just last year I purchased an Abu Garcia Black Max baitcasting combo, did my research and learned how to properly use it. I love it more than my spinning gear. I wish I knew more about baitcasters when I first tried to learn how to use them.