Wikipedia’s definition of the K.I.S.S Principle.

KISS is an acronym for the design principle articulated by Kelly Johnson, Keep it simple, Stupid!.^[1] Variations include “keep it short and simple”, “keep it simple sir”, “keep it simple or be stupid”, “keep it simple and straightforward” or “keep it simple and sincere.”^[2] The KISS principle states that most systems work best if they are kept simple rather than made complex, therefore simplicity should be a key goal in design and unnecessary complexity should be avoided.

http://en.wikipedia.org/wiki/KISS_principle

Applying the KISS principle to your design is a lot more difficult then you would expect.
You want to avoid the temptation to go overboard with your invitation design. A good designer will stick to one or two fonts, graphics, and colors colours and lots and lots of white space! You want your invitations to be unique yet not to ‘busy’ so they stay elegant and legible.

I will expand later on colors as well as typography : )

~ Ashley