Literally everything except death. However, the 'everything' there largely includes dying, so yeah.
Although the top ones are in no particular order:
Being alone forever, being alone for long periods of time if not forever, not having someone to love, not being loved, spiders, heights, paralysis, all kinds of horrible deaths, having anything happen to someone I care about and I'm not there, having anything happen to someone I care about while I was there and I wasn't able to do anything, thermonuclear war, happiness, people of the same sex as me, people of the opposite sex to me, any people not covered sufficiently by the former two categories probably in that order, spiders, spider webs, anything that looks like a spider web, anything that makes reference to spiders or spider webs, sadness, not ever catching up with reading, not being forgiven, my face, my body generally, getting any uglier, being pushed away by people I care about, being seen as genuinely bad rather than good bad by anyone in a position to make an accurate judgement, being too inferior to anyone I care about, roller-coasters, the world, not being able to fight, people in positions of control who I am unable to affect through reason or otherwise attack, growing up, being older than 17, being younger than 17, being older than 27, Yellowstone erupting, the rich and powerful, the future, not being able to play any instrument, having poor judgement, not having sufficient self-awareness, being illogical and unaware of it, disappointing anyone I care about, being really poor, being controlled, being unable to protect or provide in any way, being thought of badly by or failing anyone I actually care about, anyone who walks or stands behind me, the law, having to submit to anyone I don't want to, not having artistic ability after all, having one but not both of either the equipment or training necessary to save someone's life if it ever falls to me, spiders, conscription, idiots, my past, not having a contingency plan, any authority of any kind at all whatsoever, not being able to fuck anything up if I find it necessary, people I care about generally, being unable to garner any respect from the peasantry, anyone finding out my past whom I have not revealed it to, anyone finding out my old name once I get my new name, meeting new people who have heard of me, having my inner weakness shown around someone who I don't know or trust with it, having anyone I don't want to see through me see through me. Spiders.
Although I do have my moments where I remember that death doesn't scare me and I really do stop being afraid of pretty much anything. Those moments are nice.
They don't happen often, though. Typically when shit is really bad anyway.