Think about the size and shape of the card. Square, or unusually shaped cards can be problematic to export as postal regulations and overseas tariffs are often based on size and shape as well as weight. On the home front, unless you are prepared to provide a spinner for unusually sized cards, some retailers dismiss them because of the difficulty of display.
Bear in mind that large envelopes over 165mm x 240, or smaller cards with a thickness of more than 5mm now fall into the Large Letter category of UK postage and should have a Large Letter logo (obtainable from the GCA) on the back of the card.
Think about the top third of the design - this is the bit card buyers see on the racks. There needs to be some action there, be it design or wording. If you are using words, try to put them at the top!
Think about price, retailer will expect to make at least 100% mark up on your cards. When designing a card, especially in the case of handmade designs, carefully work out the costs involved.