The problem is not that the plane is made of composites. The problem is that the last time Boeing designed an airliner (the 777) was 15 years ago (first flight of the 777: June 12 1994).
Where do you think all the staff went between the 777 and 787? What happened to the expertise, the experience?
And when it was time to start the next project, let's hire fresh (and cheap) people with little if any experience.
Result?
Well, you see it. 2 years delay and counting.
I do not loose faith in their design capability for a final product, the certification process takes care of ensuring that the plane will be safe. But in the management that was there 10 years ago, there is very little respect left, I'd say.
If one learns best from mistakes, then they are learning. The final question is: will management have learned their lesson for the *next* plane, or will they lay-off people by the truck load yet again, losing all their hard-earned experience?