The first expectation that a user has of your product is that your product has integrity. Users expect your product to do what it says it will. For example, if your product is a lawnmower, then it had better mow some lawns, because people will be expecting it to mow lawns. If you say that you’re going to find me the cheapest flight to Seattle, well, then, you’d better find me the cheapest flight to Seattle. Your “Delete” button should delete stuff. This probably sounds simple—and it is—but there are plenty of offenders, and these offenses are likely to cause people to abandon your product for a better one.
Having integrity extends beyond your explicit claims, too. If something even just seems like a lawnmower, then people are going to expect it to be able to mow some lawns. So if your product looks like a duck and quacks like a duck, then you should make sure that it is a duck.