Operating System Design/User Ability
It is important for designers of mass-market User Interfaces to keep in mind that not all users are equally computer savvy. A mass-market operating system, such as Windows, has amongst its user base both beginners, as well as experienced power-users. What makes an OS interface more user-friendly for a first-time user may not be user-friendly to a power-user. For instance, the 'Clippy' assistant in Office may be of assistance to first-time users. Yet it is often described as annoying for seasoned users of office suite software, and in many cases may detract from the usability of the software.
As a rule, operating systems meant for more advanced users have fewer graphical configuration tools, and may have more of a focus on speed than looks. As an example, Slackware Linux has virtually no configuration tools and so is seen as a power-user's operating system, while Ubuntu has most tasks automated and so is seen as a user-friendly operating system.
Nearly all operating systems, even if they have a easy-to-use GUI, also allow users to go around the GUI to hand-tweak the configuration. "It is a significant advantage to all users if advanced users can make hand-tweaked modifications to config files when serious problems arise."
Many people have the idea of making the computer present one user interface by default to new users, then later present a different "advanced user interface" to "advanced users". This is generally regarded as a well-meaning mistake.