General:
* User-friendliness
* Simplicity
* Accessibility
* Intuitiveness
* Usability
* Convenience
* Effortless
* Straightforward
More specific:
* Learnability: How easy it is to learn to use something.
* Efficiency: How quickly and easily someone can complete tasks.
* Satisfaction: How enjoyable the experience is.
* Clarity: How well-designed and understandable the interface is.
* Comprehensibility: How easy it is to understand the instructions and information.
Informal:
* No brainer
* Piece of cake
* A breeze
The best word to use will depend on the specific context and what you are trying to convey.