I think NOT amateur friendly is the easier definition! My TB is most definitely not one who would be described as Amateur Friendly. He is, however, a great schoolmaster for me now - even though I taught him what he knows! The more he knows, the more particular he is about wanting things done correctly. Now, we could have taught him that correct was reaching forward and pulling his left ear for a canter pirouette, and he’d be fine with that - but would demand it gets pulled the correct direction and with the correct amount of pressure. He gets explosive when things are not done in the way he has learned is correct. Therefore, not Ammy Friendly. I think had he never raced or evented he would be very amateur friendly because he is sensitive but has almost no spook except for consistently being terrified of cattle. He’s pretty honest, never gets tired or complains about working hard, and learns quickly. I suspect if he had been trained as a dressage horse from the start he would have easily made GP and be much more amateur friendly than my mare except for gaits which would always be difficult to ride - they are the best TB gaits I have seen, and he has a short back which makes it harder.
Examples of amateur friendly which are very different:
I think my mare is quite ammy friendly. She will work and work and try hard without complaint. She is very sensitive and responsive, and treats aids as instruction to move a body part even if they are not for a known movement - so she is great for learning how to ride various movements. If you are too confusing with conflicting signals, she halts rather than fly off the handle, and as soon as you clarify she happily goes back to work. She would never tolerate rough treatment, but I think most amateurs are too kind, not rough, so that’s not an issue. She had some heat cycle pain issues when younger which kept her from being ammy friendly until we understood how to help manage them. Barring the many things which can happen with horses, she will end up an FEI horse because she has the energy and tendency to do FEI movements, and any amount of flubbing things up on my part won’t prevent that. (Injury, illness, etc., being the normal distractions which could prevent this.)
My trainer has a mare I call The Seatmaster. She is SUPER ammy friendly. She is literally bombproof - a transformer blew up during a lesson with a beginner and she didn’t react. She prefers not to move and if you block with your seat in the least, she will just halt. For a non-blocking beginner, she plods around like a training/first level horse and looks pleasant. If you’re learning and don’t really have things down and try to set her head, she will pull back and never go round. If you know how to get a horse round back to front, she becomes a forward ride with wonderful upper level lateral work, pirouettes, etc. She’s both a schoolmaster and an amateur friendly horse.
My mom’s mare was her last horse before she quit riding. She rarely chooses to go faster than a walk, even to get to her beloved food. She is a sure-footed and reliable trail horse. In the arena, she wants her back to swing. So depending what you’re doing with your seat she will vary from halt steps to long and low. She basically keeps herself round and working over her back without you doing anything because she learned how good that feels to her, and she’s about self preservation. If she had a competitive dressage home, I think an adult amateur could have easily gotten their bronze on her with almost no pro rides. I can’t imagine her ever happily become an FEI horse as that’s just way too much effort, and she would have been unlikely to score in the 70s. But she would have been plenty of horse for most adult amateurs of modest goals who want a multi-use horse they can play with.