I think part of it is knowing how to use libraries. It's actually a good thing to make use of well-tested implementations a lot of time rather than re-inventin

I think part of it is knowing how to use libraries. It's actually a good thing to make use of well-tested implementations a lot of time rather than re-inventing the wheel: for instance it would be crazy to implement your own cryptography functions, or your own networking stack in most cases. Libraries are good when they can encapsulate a very well-defined set of functionality behind a well-defined interface. Even better if that interface is arrived at through a standards process. To me, where libraries get a bit more questionable is when they exist in the realm of pure abstraction, or when they try to own the flow of control or provide the structure around which your program should hang. For instance, with something like Ruby on Rails, it sometimes feels like you are trying to undo what the framework has assumed you need so that you can get the functionality you want. A good library should be something you build on top of, not something you carve your implementation out of.