Architecture, for sure. Learn how to start a thing the right way, with a clear horizon and how this thing could scale is a matter of experience. You can learn some key points on books, but till you reach the perception of what points you should prioritize thinking on the customer, how this points may vary for different customers, and how it could drive you architecture, etc... Think in what problems you could avoid just making the right choice between two options that seems both ok. Man, thats a matter of experience.
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