Dressage at the most base levels is simply good riding and training practices. A horse and rider with a good foundation in any discipline should be able to do a respectable training or first level test. Most horses and riders will "max out" in dressage at this level, however schooling up to about first level dressage will help any horse and rider out. These levels require basic obedience, balance and connection.
The low-mid levels of dressage, second and third level start to require collection, and a more dressage focused training program. The mid levels, fourth and PSG, are very difficult to get to and rewuire far more collection. The upper levels I2 through the Grand Prix Special require the highest degree of collection in competetive dressage and very, very few horses will ever make it to this level.
They say money doesn't buy happiness -- well happiness doesn't buy horses!