We've got fundamental scientific theories (such as quantum theory and relativity) that test out superbly, even if we don't quite know how they all fit into a whole, but we're hung up trying to understand complicated phenomena, like living things. How much complexity can we handle?
We ought to be able to use computers to model complicated things, but we can't as yet write software that's complicated enough to take advantage of the ever-bigger computers we are learning to build.
Complexity, side effects, legacy. How much can we handle? That's the question of the new century.
There's a social variant of the same problem:
In the twentieth century we become powerful enough to destroy ourselves, but we seemed to be able to handle that. Now technology and information flow have improved to the point that a small number of us might be able to destroy us all. Can we handle that?