I think in addition to the styling of the SI nameplate, the overall layout of the covers also has some highs and lows. In general, I like clean covers with not a lot of extra writing on them, especially when it does not refer to the cover itself. Also, any pre-barcode covers get a slight bump just because they don't have that aesthetically worthless symbol taking up space.
In general, I tend to like action shots on the covers and definitely want the athlete to be in uniform, not street clothes.
But can I list exactly what makes a great cover? No, but I know it when I see it.