Let's say you have a value written into the Country field in your existing org that, when enabling the State and Country as Picklists feature, is actually a State. A good example is some of the US protectorates, like Guam or Puerto Rico. If you're converting really old data, you will definitely find a lot of these values in Country fields.
I'd love for the system to detect that a value found in the Country Conversion process is actually a State value. it should the give me the ability to map that existing Country value into one of the standardized State value, and in the same action, select the appropriate Country to which the standardized State value is mapped/
Otherwise, I'm left to move away from the tool, manually search for all the data that has States written into Country fields, and correct those manually / through Data Loader / etc. That's very labourious, and slows down the process of enabling this feature.