Currently a formula can look plain and a bit boring.
I would suggest highlighting the syntax words like Case, When, Then, End etc.
The result should look like this
Talking about visualization ... This will differentiate user-input texts from Tableau-defined syntactic words. It will make the formula easier to read and to debug.
Update: I would also add that we could highlight the quotes so that it becomes
When 'California' Then 'CA'
Update 2 on 1/11/2017: When there are multiple opening brackets, we may not know how many closing brackets are needed.
So if we color code the brackets, it may give users hints on how many closing brackets are missing.
Window_Max( if Attr(Category)="Technology" then Sum(Sales) end)