Just before importing, display example table of import data fields
Display 5 or 6 examples lines of imported data, BEFORE actually importing. Reason: Somehow the phone numbers failed to import, even though I thought I'd checked everything carefully. Seeing a table of sample data for several records would have clearly shown me that the phone number column was empty.
Randy Rensch commented
When importing records, check for invalid email addresses BEFORE the import, not after! In fact, it would be easier to have the whole file rejected, so I can fix it, rather than have to manually import the rows that were rejected!!