I've recently built an interest with sugar gliders. Just wondering, what are some of the things they are afraid of?

after reading a glider book. anything smaller then them is food. anything
larger then them is a predator. this is why it can take some time to bond with
a glider. if not held at a young age it can take time to break thru that natural
defense instincts.

I second the above. It can sometimes vary depending on glider... but anything new and unusual is suspicious. Gliders can be the most LOVING and PLAYFUL things once they're comfortable. But they are still basically on the bottom of the natural food chain, meaning that anything slightly out of the ordinary is to them: deadly and must be feared.
Whether that's a towel, brightly colored water bottle, or mama's hand in a surprising place comes into question AFTER the initial "woAH?"