Very interesting Jaydee... around here, I don't even think classes are divided up into ponies vs horses, just ages and levels. But what do I know :)
And yes, I agree, being perched up high is a good way to fall - and you fall from a lot higher! That's why I like 15 hh. My BO advised that if she wants to compete, she should aim for around 15.2. 15 is on the low side, but perfect for us shorties. I would be a bit nervous letting her jump on a 16 h + horse, but lots of people do it here.
Honestly, I don't really care for the pony temperament. I'm sure some of you will disagree strongly and I'm sure there are terrific ponies around there, but all the ponies we've had her on, including one we leased, have been cantankerous and stubborn. I like this horse's temperament because he's got the level-headedness of the QH with the inquisitiveness and intelligence of an Arab. That may get him in trouble (ie, he's an escape artist), but it makes him a quick learner and a fun horse to be around! For his age, I'd say he's doing well to be a steady as he is.