Well dressage in NA is quite a young sport... I would say it has only really been around in the greater part of NA since about the 60s or 70s.. so the development that has happened is basically all just "catching up" to Europe - which is still going on. We are just now getting some very good horses that can compete with the European's horses... but we are still lacking hugely in rider education. I know most of the top riders were either born and trained in Europe or travelled there to train. My coach who has ridden in a few Olympics and a WEG was trained in Europe in the 70s and 80s as there was actually no one doing dressage in her area in NA at the time, she was the first. As Shasta has pointed out, we have a huge lack of well schooled horses as well. In Europe PSG horses are churned out through a well oiled machine... here our top riders are too busy, the financial burden is too high or they even don't have the skills.. Mummy and daddy bought the international dressage horse and they couldn't train one to GP if their life depended on it and have never sat on anything under 7 or 8..
As far as development in Europe over the last years - I agree wholely with Spyder, the horses have changed immensely and are no longer amatuer friendly. There comes a point where riders must now decide their "fate" in the sport with the lines they choose as something without enough "zing" will not take you anywhere in international dressage anymore. And the ones that do make it in international dressage are rarely ones that can be retired to an Amatuer... they are still barely broke even after the GP and require a very sensitive, compassionate rider.. I can't imagine myself being able to hop on Salinero, Totilas, Valegro, etc.. and put together a semblance of anything really, where as horses like Rusty, Ahlerich, etc.. I probably could have hopped up on, although they were hot horses as well!!
The judging is the same, a 60% is still a 60%, the riding has had to improve in the use of tact and stillness in the riders and training methods have become far more sensitive, I think. We understand biomechanics better, building the horse as an athlete, nutrition, maintenance of the horses' soundness and veterinary care are better and so is the technology in the fit of the tack and materials used... So all in all I think it is better, the horses are happier and better cared for and the competitions have become way, way tougher. Talking to people that have been around for a long time, like my coach or six-time Olympian Robert Dover or FEI 5* judges like Axel Steiner is very eye opening to me.. I think it is unfair for people on forums like these to be judging the sport so harshly having never experienced the top, or talked to those at the top and whom have been there for a long time... It is tough to get a good view of a family just peeking in the window at odd times...
They say money doesn't buy happiness -- well happiness doesn't buy horses!