I have a more general question for you. I'm working in Parallel Data Structures and Parallel Algorithms. That is a nice topic with a lot of interesting challenges. However, I have some problem to argue for why is necessary use parallel solutions. The most common justification is "We need to take advantage of the current architectures", "Parallel alternatives speed up sequential alternatives", but it isn't enough. Some people can think: OK, I believe you, but I don't have any problem waiting 10 minutes for my efficient sequential algorithm instead of 2 minutes for your parallel algorithm. That reflects a weak spot of parallel solutions: they are, in general, difficult to program and need more time to debugging.
So, now I'm trying to find a better motivation for parallel solutions. I have this motivation now: There are problems that don't have efficient solutions in sequential, and parallel solutions appear like the best alternative for those kind of problems. My question now for you is: How characterize this kind of problems?. I thought in problems with a lot of computation, like some graph problems, but I would like to know other view points.