The WDL is great for adding more capabilities to the Flows, but I still feel constrained by its limitations.
I'm not sure if the Flow team has influence over the language or can talk to the team that maintains it, but more functionality needs to be added for simple actions that are common in other languages and necessary for conditions, filters and data manipulation. A couple starters:
Join an array: join(array, optional connector string) outputs a single string
DateTimeDiff to find the timespan between two dates: datediff(datetime1, datetime2) outputs difference in seconds or milliseconds
I'm sure there are more that could be added that might be easy fixes for some of the issues other people have on the forums.