Good design is innovative. Good design must be useful. Good design is aesthetic design. Good design makes a product understandable. Good design is honest. Good design is unobtrusive. Good design is long-lasting. Good design is consistent in every detail. Good design is environmentally friendly. And last but not least, good design is as little design as possible. Less, but better.

Dieter Rams

Principles of design:
1. Use both knowledge in the world and knowledge in the head.
2. Simplify the structure of tasks.
3. Make things visible: bridge gulfs between Execution and Evaluation.
4. Get the mappings right.
5. Exploit the power of constraints.
6. Design for error.
7. When all else fails, standardize.

Donald A. Norman

Product successes and failures have shown repeatedly that users don't care that much about features. Users only care about achieving their goals.

Alan Cooper, The Inmates Are Running the Asylum: Why High Tech Products Drive Us Crazy and How to Restore the Sanity

User-centered design means understanding what your users need, how they think, and how they behave - and incorporating that understanding into every aspect of your process.

Jesse James Garrett

Designers need to be more than ambassadors, they need to be fully functioning and fully aware members of strategic decision-making teams in a company.

Nathan Shedroff

Winning products come from the deep understanding of the user’s needs combined with an equally deep understanding of what’s just now possible.

Marty Cagan

I think it's more accurate to think of aesthetics as a key ingredient in a recipe, as opposed to the icing on the cake.

Stephen P. Anderson, Seductive Interaction Design: Creating Playful, Fun, and Effective User Experiences

Mobile users will do everything desktop users will do, but it must be presented in a usable way. So do all that you can to make sure as many people as possible can access your content and functionality.

Brad Frost

Simplicity is about subtracting the obvious and adding the meaningful.

John Maeda

A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.

Douglas Adams

Design can be art. Design can be aesthetics. Design is so simple, that's why it is so complicated.

Paul Rand

Sketches are social things. They are lonely outside the company of other sketches and related reference material. They are lonely if they are discarded as soon as they are done. And they definitely are happiest when everyone in the studio working on the project has spent time with them.

Bill Buxton, Sketching User Experiences: Getting the Design Right and the Right Design

What many refer to as intuition, then, is not the untaught or unteachable but instead is a learned understanding and respect of process, molded by experience and refined over a great deal of time and practice.

Jon Kolko, Thoughts on Interaction Design