The dashboard filter widget seems to support structured data: Single Select, Multi Select, Assignment, Yes/No, Status, Linked Record.
Like the filter in the report toolbar allow us to use "Lookup" fields of these other field types in the filter widget.
For a use case, an account rep is assigned to the account and tasks are generated against the account. We want to be able to filter the account rep on the tasks grid, so we know who the primary contact is.