One of the most potentially tiresome elements of any story in literature, film or theater is exposition -- the delivery of information that helps the audiences understand the plot or characters but doesn't exactly thrill. The only error worse than relying on extensive exposition is stuffing all that dramatic deadwood into the beginning of the tale. Over-explaining is a surefire way to alienate readers or viewers who are there to be emotionally grabbed up and swept away -- and it's just as ineffective when it comes to marketing your products or services.
Does this mean that detailed information is inherently bad? No! Sometimes it's those little details that make what you've got more desirable than the other guy's offerings. And if you're marketing a house, car, boat or some other high-dollar item, you'd better have reams of data on hand for the careful consideration that goes into such purchases. In other words, the harder the sell, the more explaining you have to do in your marketing content. But even then, you have to think about where and when all this content needs to come up in your audience member's journey through the sales funnel.
In another post I talked about the pitfalls of the "endless home page," in which an entire website's content seems to be shoehorned into the very first screen the viewer encounters. This is an extreme example of over-explaining, but it can and does happen. Every word of that information may be valuable -- some of it may even make a major contribution toward an eventual purchase -- but it doesn't belong here. That's partly because the home page needs to present a welcoming, non-intimidating gateway to your brand, and partly because the home page is mostly about the sizzle, not the steak. Your marketing content's first task is to engage viewers on an emotional level, not an intellectual one. The initial push of excitement, wonder and/or relief you make here will carry the viewer on to the next level of discovery, whether that involves turning a page in your pamphlet or clicking on the right link to answer a burning need to know more.
At this point you can start introducing more and more detailed information because you know you've already got your visitors half sold. If you've done it right, they'll now focus on the data with the goal of confirming their initial hopes and excitement. Even the skeptics will come at the information hoping that it will prove them wrong about their doubts. Bear in mind that some of the many details you provide may provide certain visitors with reasons not to buy your product or service. If you haven't already got them on your side emotionally, you'll lose them then and there. But if you have, then they might keep pushing forward anyway to find other data that outweighs their objections.
So more often than not, "over-explaining" is really a matter of "premature explaining," or putting the intellectual cart before the emotional horse. You or your copywriter may compose marketing content bursting with sound, sensible, logical reasons to buy something -- but ultimately it's the primitive, emotional "I want this" impulse that compels the actual decision. Before you appeal to the brain, make sure you've already appealed to the trigger finger!