Can the qualities be measured or categorized?
What are the similarities and differences? What is the comparison? Visual differences can reveal different inputs, outcomes, or impacts.
What are repeated ideas, concepts, or conceptual categories?
One example would be disaggregating a body of data by demographic categories such as sex, race, income, or migration status to show disparate impact of government policies on different groups, or to explore disparate outcomes such as under-5 mortality or school dropout rates.
Using multiple visualizations to illustrate different states can also help illuminate differences.
Note, once categories are created, they have a tendency to remain stable and can influence further analysis, as changing categories complicates the examination of change over time.