The Hermes Message System has evolved in response to the needs and criticisms of users. This paper gives examples of some less than successful features, many of which have been changed, so that future designers will know what didn't work, as well as what does. Principles derived from this experience are: (a) What you see should be what you can type. (b) Commands and syntax should be uniform. (c) Commands and objects should be organized into groups. (d) Hierarchy is great for organizing things you know about but much less useful for finding things you don't know. Even with careful design, it is impossible to predict what users will dislike so it is important to design programs so they can be easily changed.