Clarity is vital to an effective business card.
But clarity, although necessary, is not sufficient to really set your card apart from the rest.
If clarity were the only consideration, then the most effective business card imaginable would be a white rectangle that contains your name, your business, and your contact information, and nothing else.
If you were handed a business card like this in the real world, you'd more likely than not throw it out without even a second look.
That card contains everything a potential client would need to contact you, yes--but why would they?What incentive have you given them? This brings us to the point of designing an effective business card: designing for communication.
If your goal is to bring in new clients and valuable contacts to help your business grow, then your business card needs not only to make your contact information available, but also to communicate to those potential clients and contacts exactly what separates your business from your competition, and what special skills, experience, or ideas you can bring to the table so that you can benefit both yourself and your clients.
The best way to do this depends on just what your business is.
One general principle, though, is to work in terms of images whenever possible, as opposed to text.
A client's interest is then piqued: he or she sees what you can do, and how well you can do it.
The client then has a much greater interest in contacting you, since he or she knows that yours is a proven talent.