it really depends on the circumstances of the flight. LAX-MIA has been operated by several different aircraft types over the last few years, including 2-class 767-300, 2-class 757 and 3-class 777 aircraft.
If you had a 3-class 777, then it was operated as a 2-class service so the business class seats are given to some economy class passengers. The service is still economy class, just a better seat.
If you had a 757, then the first class seating is similar to a Qantas 737-800 business class. There is no business class on these aircraft. You would have received two feeds between LAX and MIA, being a full meal and a snack.
If you had a 767-300, then you would have had a seat similar to the 777 business class (i.e. lots of leg room and recline action) and the same meal service as the 757.
If you didn't find much difference between economy and they way you travelled, then I expect you were seated in the business class section of a 777 and received economy food and service. If that is the case, then its not really an upgrade or a business class experience, but it a very pleasant way to experience a long economy flight.