The most difficult part of TDD: You have to write tests first before coding/design/implementation

"Have to actually think what you expect from a piece of code BEFORE you write it! Madness! Instead of just writing code, you have to think before you code! And if you can't formulate properly an expectation, HOW do you write the code in the first place?!"

http://www.sapiensworks.com/blog/post/2013/07/17/-TDD-Is-Not-Slow.aspx