I just like to add that the database design is the real secret sauce, important even more than external APIs in my opinion.

This is something that I've stumbled into as well. DB models AND dataflow. Getting both of those well spec'd makes things a lot easier.

Well, not DB design really, you can achieve the same thing by defining your POCOs well. I switched entirely to code-first design years ago. If you haven't worked with a good ORM, you're really missing out, though I admit there was quite a bit of friction at first.