New "Locals" feature where you can now define a variable that is scoped to your current transformation allowing you to reuse logic that used to require you to copy that formula multiple times inside your expressions. Another current technique is to use Derived Columns to generate synthetic columns that were not meant for loading into your destination and were used in this "temporary" way. Now you can use locals to store data that won't propagate outside of the current scope.
Now you can edit directly inside Derived Column transformation text boxes without the Expression Builder automatically launching. Typing or editing your formulas directly will also include standard validation and enables copy & paste without needing to open the Expression Builder. This means that the Clone button has moved to the top of the column list as well as a new Open Expression Builder option.
We are closing in on our 1-year anniversary of the GA of Data Flows in ADF and we gathered a ton of feedback from our community that we consolidated into these updates including making the experience full-screen and adding inline function help. We hope that you will find these changes make for a more productive and enjoyable experience in ADF data flows as we've recognized that the bulk of time spent in ADF by our data flow ETL developers ends up being inside the Expression Builder.