A UI should be consistent with its overall environment. Consistency with metaphors and the users' conceptual model. Simplicity KISS - Keep it simple stupid. Simple things simple. Hide functionality from the user until they require it.
Humans are very good at finding out how things work through exploration and discovery.
For example, consider how people learn how to work the controls of an unfamiliar automobile. They explore the interface and are generally unafraid to try out things. UIs of computers need to support that type of exploration and discovery. Warn about potential data loss. Make actions reversible or recoverable.