This article talks about consumer software, but most of the issues are applicable even to relatively technical enterprise software. The end users of this kind of software generally have a lot of expertise in their daily routines, but stumble when they are asked to use the software to do something outside of their typical use patterns. It's important to stay humble and realize that the software you're offering to a customer is, in all likelihood, going to have a relatively small (but hopefully positive) impact on the user's life, and in these cases the users are not going to want to spend a lot of their time mastering your software.