Angus is a British breed of cattle. The British breeds are known for being smaller and finishing earlier than most other breeds (you can make them fatter at a younger age). Angus certainly fall deeply in to that description as a breed (smaller earlier finishing). Beyond the obvious morphological differences, I have never been able to tell the difference between Red and Black Angus in terms of performance in the pasture or on the lot (or on the table for that matter). British breed beef can be wonderful, early maturation allows for good fattening qualities, and if the money is spent to get the finish on them, they make a great eating animal. However, as Kevin said, the animal is made good by genetic potential AND environmental input. The animal finishes fatter when it is fed correctly. The fat ends up as marbling (as opposed to inter-muscular fat) based on the breed and the specific animals genetic propensity to store its fat that manner.
Some breeds are almost impossible to get to marble up. They just won't grade. The British Breeds do. Lots of other breeds do as well.
Feeding cattle ends up being about economics. It costs a lot to get cows to grade higher at slaughter. It's a simple as that. the question is, will folks pay more for better beef? I do. I'll pay the extra dollar or two a pound they charge to go to a higher quality cut. Unfortunately, in the USA, we tend to food shop by bottom line cost rather than quality per price. It's a shame, and hard on the producer. the trend was changing a bit, but I'm afraid this recession will shove us back to choice cuts sold cheap at the grocery with little opportunity for better beef.