Usable interfaces are all about productivity for those that use your software every day. They feature a UI that anticipates you, but gets out of your way and ensures that your mistakes are recoverable.
Novices require more hand-holding. They may either be new to the application and not yet a power-user, or they may be an occasional user, or they may be watching a demo. In each of these cases, the application must be easy to learn and understand.
When you prepare your user profiles for a UI Design or a product requirements document, it may seem that these two user types are hopelessly irreconcilable, and sometimes you’ll specify two completely separate UI’s (e.g. wizards in addition to the “normal” UI). Sometimes this is appropriate, but on his blog, Philip Brittan discusses a product we worked on, where features supporting both types were implemented with the same UI.