8 answers
If you arent hell bent on "tiers" how about points?
Formula field with Number Output:
IF(NOT(ISBLANK(FirstName)), 1,0)+
IF(NOT(ISBLANK(LastName)), 1,0)+
IF(NOT(ISBLANK(Phone)), 1,0)+
IF(NOT(ISBLANK(Email)), 1,0)+
IF(NOT(ISBLANK(Street)), 1,0)
Then the higher the number, the more complete the Lead is and you can run a report looking for Leads where this field is < 3 for less complete Leads.