The new "Show Missing Values" feature in Tableau 7 allows you to where there are missing dates or numbers in your data set: http://www.tableausoftware.com/new-features/7.0?quicktabs_9=2#quicktabs-9.
This is a great feature, but it would be even better if you could show missing values for all data types. For example, it would be great to see a bar graph of all 50 states, even when only 48 states exist in your data.
It would be nice to show all values in two different ways:
1. Allow you to enter what the domain should be for any field (Right click on field, Field Properties -> Domain...)
2. Scan the entire table for a complete list of values, similar to how the Show More/Fewer button on quick filters work.
For me, #1 is much more important.
I accomplish this task currently by doing complicated database work on the table that Tableau uses, but it would be much more efficient to have Tableau do the work.