Seems to me that enhanced economy products are not necessarily a good option for an airline and was largely the result of the temporary boom in business travel in the 1990s. In terms of cost structure, I would believe that the more classes you have the more complex stocking and cleaning the plane becomes and the more differentiated training you would need for flight attendents for each classes. I was amazed when I flew United internationally that they still have three fully differentiated classes. British Airways now basically has four in the sense that they enhanced economy product has a completely different seat design.
I think KLM, Continental, Delta, and Northwest have it right in offering a very good business class product (well, maybe not NWA as much) rather than first and business. I would much rather have an airline offer a great business class seat and a good coach product. That seems to be Continental's philosophy in particular.
Somewhere in the trade press I also read that the more you make coach or enhanced coach comfortable the less attractive it becomes for business travelers to pay out for business class and such. In this time of reduced business travel, it seems to me that UAL, AA
and others should be streamlining and rationalizing their in-flight services to reduce costs and enhance profits.