Well areas like Herfordshire and Cambridgshire, as well as parts of Essex are very affluent areas and with them all having Stansted as their local airport, they are faced with easyJet and Ryanair as their only choices. There is certainly enough demand for full serive and long haul services from Stansted, but the rail connections and BAA's managment of the airport has held it back.
I would say that the flag carriers, especially for long haul travel can do well at Stansted and we have seen some examples of it in the past. The major reason why the flag carriers find it hard to use Stansted is because of the onward travel to London. I'm sure that if Stansted was more accessible by rail from London than say Gatwick, we would start to see the shift of some traffic into Stansted.