Something clean, minimalist, that looks like it was "designed" and not just "purchased". Something that people would want to keep just because it looks so damn cool. Something that would win a design award!
I'm open to exceptionally creative themes. Vintage. Typewriter-fonts. Textured. Hipster.
Negative space (whitespace) is awesome.
Happy to vary the physical shape of the card too - circle, vertical, foldable, transparent.
Either black/white/grey, or a super-bright block colour.