The importance of mulch. The general utility of neem oil. That fish fertilizer works great and is very forgiving for the novice. How much difference picking the right cultivar for your bio-region can make.
I've only been gardening for 4 years and it feels like I've learned a lot, but that the plants have so much more to teach me.
when planting a potted plant into unprepared compacted soil, dig a hole twice the diameter of the pot and at least 1.5 times as deep, then mix removed soil with compost and use that to fill the extra gaps.
You can't know everything, but someone knows what you don't. Build a community and talk with people - ask questions and form relationships with other gardeners because the passion you feel is shared among many others.