That's the most effective billboard one can imagine, particularly if it's typeset properly and if the coffee is good.
Most billboards aren't nearly as useful, because the wrong service is promoted, or, more likely, because someone saw all that space and worked hard to fill it up.
The same thing is true of most websites. You know so well the why's and how's of what you built and how terrific it is, and the thought of using just a few words when a bunch will do is frightening indeed.
No, your solution doesn't have to be simple or obvious. But the story about what it accomplishes does.
The goal of a marketing interaction isn't to close the sale, any more than the goal of a first date is to get married. No, the opportunity is to move forward, to earn attention and trust and curiosity and conversation.
Simple, clear and actionable.