The v10 formula configuration window had two very small advantages. First, it always had an extra 'line' for another output field (no pressing '+' required). Second, it defaulted to letting you immediately begin typing the name of the next column (no need to press 'Select Column' then 'Add New Column'). I know these are minor, but every little thing counts when you're doing heavy development.
It has been brought up that the following comments were given during the beta. While I appreciate the reasoning of requiring 'obvious intention,' my personal opinion is that it is overkill in this scenario. Even for new users, the old design was quite intuitive.
"Thanks for taking the time to provide feedback! This touches a conversation topic that has been ongoing here at Alteryx. While we want workflow development to be as fast as possible, we also are trying to address the overall usability of the tool and make sure it is very clear what we intend the user to do. We decided to have the UI ask for an explicit action (pick an existing field to edit or click to add a new field) to help make those options clear, as we have found that users don't always understand from the existing tool that this is the first decision they should make when using the tool. That being said, your feedback is definitely valuable. I will be sure to bring this up as we are making improvements to the new tool and see if there's a compromise that we can make on speed vs. obvious intention. Thanks for taking the time!"