it is a very opinionated topic, but there again I think the horse world is very opinionated and thats because people have passion for horses.
There have been a few comments about the clarification of
'agression' let me just explain the level of agression the lady I rode for used with her horses.
(We can call her a lady for now but I'm not sure its accurate

)
This lady has a large herd of horses and I know that she loves them to bits, she rescues quite a few from gypsies, markets etc.
However, I wouldn't ride / handle horses as she wanted me to. She used to tell me off for being too soft with the horses, here's some examples of her 'training' methods:
Horse puts head up when putting bridle on: solution =
smack horse on head with bridle.
(I worked with this horse for just two weeks and eventually it trusted me and put its head down to have the bridle on - just though patience, calmness and reward - she came along and tacked her up once after that and the poor horse was back to square one again.)
Horse backs up / shys because its scared of something (lady is on ground with horse): solution =
kick horse hard under its stomach (this was the horse I rode for her and I couldn't stop her, its her horse after all)
Large (very overweight horse) pushes lady over in field / is too bolchy: solution =
tie horse up on short rope and hit with schooling whip, that will teach it.
Young filly won't load in trailer: solution =
get someone to pull with rope from inside trailer while you whip the horse's back end, the cut legs and petrified expression on the horse is all part of the training.
Do you get the picture?

Do you see why I had to leave? I loved the horse I was riding to bits, but if I said anything about the way she handled her horses, she exploded on me and I was worried that I wouldn't see the horse again. The horse I rode wasn't completely sound he wasn't forward going and I was quite happy to just use leg and encouragement with my seat and voice, he wasn't 100% afterall. But I used to get that 'lady' shouting at me for not hitting the horse with the whip. I am quite happy to flick the horse on the bum to wake him up and tell him to listen to my aids but she wanted me to smack hard, to HURT the horse - no can do I'm afraid

.
Even though I have had to now stop riding because I left (until I buy a horse), I feel much better because I used to come home stressed and upset and horses should be enjoyable! She did all of these 'agressive' actions when SHE LOST HER TEMPER - SHE was bullying the horses because she couldn't keep her cool. Pathetic.
I went to see Parelli at the weekend at the NEC

, now he
does make sense. He takes his methods from how the mare teaches and communicates with the foal, you can see that his horses and handlers are so connected and SO HAPPY to be with each other. Thats more like it.