What downgrades a die-cut from NM?

dings: A ding this serious downgrades an otherwise high perfect card to Ex+.

border creases: These are small creases in the border of the card, usually at thin parts.  Any border creases and the grade drops to Ex+.  
smudges:  A minor smudge on the back lowers the grade of an otherwise pefect card to NM- at best.  A more major smudge would downgrade to EX+ or lower.
curling:  A mint card must lay flat on the table.  If it curls that lowers an otherwise perfect card from mint to NM.  I was able to stand this card on its side without support.
popped perfs:  Popped perfs are death blows.  That lowers the grade to Vg+ at best. 
dirty backs:  A back should be clean.