I have one answer: the ETOPS limitation over remote or polar regions such as e.g. the (discontinued?) SQ21/22 SIN-EWR flight.

But the A380 flying over areas where also a 77W flies wonders me as the capacity of an A380 is only slightly more than a 77W.

I think that for that reason modern aircraft like the A340 and A380 are still four engine while aircraft with such capacity can easily run on two engines.

A 77W has a thrust of 520kN per engine (http://en.wikipedia.org/wiki/77W) makes 1040kN total and an A340 310kN per engine (http://en.wikipedia.org/wiki/A380) which makes 1240kN total, so not much more.

What are your ideas ?