Wow, thank you so much for taking the time to not only read through but also leave such detailed feedback! I can't express how much I appreciate it! And I'm so glad you like it, too!
One of the projects I had in mind was to write what I tentatively titled the "GUTS+: Tome of Helpful Examples" that would do exactly what you asked, but I haven't been able to dedicate time to it yet. A table of unique qualities would definitely be a good thing to include, though. It's typically just something about your character that doesn't necessarily always have a clear function.
Here are a couple examples for you in the meantime: one of my players is playing a handiman and two of his custom qualities are "Electrician" and "Woodworking". So far, he's used Woodworking to get additional dice for hacking through a wall and and Electrician to try and get any advantages possible on nearly anything with wires. Another character is an American football player fresh out of high school who has the quality "Big-Boned", which he's used to his advantage tackling people in combat. Yet another player's character is a ranch hand who's good with horses and is obsessed with the paranormal/cryptids, so his qualities are as vague as "Horses" and "Cryptozoology". I've allowed him to use Horses to try to interpret any animal interaction and played off any failure as "well this animal is nothing like a horse" and Cryptozoology has similarly enabled him to interact with things that aren't animals in a similar way or to see if he's heard of anything like the unknown creature before.
Thanks for your kind words! Yes, doubles came about to combat the inevitable bell curve that adding more dice to this particular system would cause. And thanks! I'm pretty happy with the Assistance system in that regard, too! It's not super reliable, but it rarely hurts to try it and when it does work, it can be pretty helpful! I'm personally a fan of when your players come up with excuses for which qualities could be helpful and how they're helpful lol.
I'm glad you found Essence well explained! I tried to do my best to make it understandable, so I'm glad it worked out. The thing about Essence as it currently exists is that Essences can't really combine or mix, but I do get what you mean about maybe there being some kind of interaction when they touch... I'll have to think about that! I'll at least add something to the GM chapter about considering what might happen if different Essence types interact. As for multiple Essences acting on a single object, as a GM I'd say it depends on what the action is. If 2 Essences are trying to Push the same object in opposite directions, then I would personally require a Contest roll using the Essence quality to decide who prevails, whereas 2 Essences trying to manipulate someone's memories or feelings would just compound. If those memories or feelings were conflicting, then the GM should describe what that conflict of "this happened and it didn't happen at the same time" would feel like in the unfortunate target's mind, or maybe one affects it slightly sooner than the other so it's a very quick sequential change?
Anyway, I've just written an equally long response! Thanks again for your kind words and for reading through the rule book at all! It really means a lot, and I appreciate it. I hope you have fun if you and your friends end up using it! :D