If I would have to choose to keep only one of my answers on StackOverflow, I’d choose this one. 

Probably, not necessary explanation

On a technical side, what is the greatest test automation challenge? It’s not programming, and not even learning about tool’s object model or application’s API.

But it’s two-fold.

1. Automated test design

2. Test Automation Framework design

And your approach to these two predetermines the way you create your automated tests.