Interesting thread.
When you have your gear sorted out and you have read the Tree Climbers Companion untill the print blurs across the page, put on your kit, enter a tree and vertigo will blow your mind. You will forget all you have crammed into your head and lock up. So before you do this, set up a cambuim saver in your shed, garage or a tree with a limb no more than double your height and sit in your saddle a metre off the ground. Then bounce for all your worth. Learning to trust your gear is vital in the tree. When you get that cramp of fear that turns your guts to water and your legs to jelly you will need all the help you can get.
I love heights. As a kid I climbed everything from trees to rocks and buildings too. Still I have felt fear in a harness when things go wrong. Assume you will be tested and put in your "apprentice" time on the ground, then a metre off the ground, then 2 metres, then 3 and so on.
Learn to let go of the tree.
Use a simple but reliable friction hitch like an English prussik.
Overengineer everything.
Climb with a buddy. Actually this rule should be at the top of the list. Never ever ever climb alone. Even if you are fooling around in your own yard at very least have someone with you who can talk you down or call 911 if you are stuck beyond self rescue.
Climbing trees is no more dangerous than racing cars, parachuting or scuba diving.
Welcome to the asylum.