I think everybody has made some good points and covered warmbloods vs. other breeds pretty well. So I won't add to the technical side of it. I can however, give you a personal account of the warmblood vs. other breeds in dressage.
I started dressage on an Appaloosa. Yes...Appaloosa. He was very talented for his breed and had fantastic movement. He certainly did not have the conformation (nor the mind) for the work though. We competed a solid second level and were training third when he had an unfortunate injury and didn't recover. Overall, a solid horse but wouldn't make it past an average 4th level. After he was injured I didn't have a horse really...so I picked up my pony. A 14.2 hand grade pony, with the crestiest neck ever lol. I'm almost 6 feet tall. I trained him up to 3rd level in about a year and a half (he was already started) and competed FEI Juniors on him. Easy mid 60's scores, and won regionals against a bunch of already proven FEI *Warmbloods*. I competed him 4th level the year after and was lucky to get a 62%. He's a special guy, who is very smart and easy to train. Could do PSG...but certainly doesn't have the movement to do it. It was a struggle to get him to do what he did - but he did it!
Fast forward to today. I have -the- most talented horse I've ever ridden, and probably the most talent horse I will ever have. He's a warmblood. Backed at 5 and a half, and without pushing him, he's now training at an easy 3rd level. He's turning 7 next thursday. I plan on PSG with him next year. Simply because he can
do it SO easily
So, for me, I know why I searched high and low for that special warmblood
. Although my pony and appaloosa could do it, it was a struggle and it was hard on them...my warmblood...so easy. Now, I realise how blessed I am to have a horse that when I say "extend", he asks "how much?" and with ease, and that not all warmbloods are that compliant.
But that's my novel on warmbloods vs. other breeds, and why I love my warmblood. Sorry for such a long message!!