When simple things need pictures, labels, or instructions, the design has failed.
-- Donald Norman

THINGS

1. Things that don't work the way you expect
2. Different things that are too similar
3. Things that are hard to see
4. Things that don't work well together
5. Things that get in your way
6. Things that are hard to handle
7. Things that are hard to remember
8. Steps that are hard to remember
9. Things that don't fit you
10. Things with "ergonomic" designs

DISPLAYS

1. Different displays that are too similar
2. Displays that look like controls

CONTROLS

1. Incompatible mapping of controls to devices
2. Unexpected placement of controls
3. Controls that work in unexpected ways
4. Controls that are too close together
5. Inconsistent control activation
6. Controls that are hard to figure out
7. Controls that are hard to remember
8. Controls that are too far away from devices
9. Unnatural ordering of control settings
10. Hidden controls
11. Too many controls
12. Unexpected mapping between functions and controls
13. Controls that are easy to activate accidently
14. Controls that have unexpected functions
15. Controls with conflicting cues
16. Controls with ambiguous labels

SIGNS, NAMES & LABELS

1. Ambiguous signs
2. Different names that are too similar
3. Patterns of signs
4. Missing labels
5. Signs that are hard to see

But there's a great deal more to it than that. Consumers seldom use an engineer's thought process before buying a product. Consider:

1. Attractive products sell better than longer-lasting, more fully-functional ugly ones.
2. Products that are uncomfortable to use won't sell well -- they'll make customers dread using them.
3. Customers expect certain aspects of appearance and functionality apart from what you need for a product to function. If you change what they expect without their assent, they won't be comfortable with the product. This includes characteristics like shape, color, and "feel."
4. Customers make buying decisions not on actual facts, but on perceptions. It's not enough to have a well-designed product; you have to have what the customers believe is a well-designed product.

Norman lists several characteristics of effective design:

Visibility	users can view an object's current state and recognize alternative actions to take
Mappings	the presentation of alternative actions is consistent with the results of those actions
Feedback	you know you did what you did
Reversibility	mistakes aren't costly
Constraints	physical, cultural and logical contraints are employed to prevent typical mistakes users make

Remember what Douglas Adams said about the thing that couldn't possibly fail: "it's impossible to get at and replace." Consider this example:

• a motor manufacturer conveniently slaps ratings on the inside of the motor casing
• the machine designer conveniently fastens the motor inside the casing
• the maintenance staff can't READ the ratings without unfastening the motor!

References

Darnell, M. www.baddesigns.com
Casey, S. Set Phasers on Stun. Aegean Publishing, 1998.
Norman, D. The Design of Everyday Things. Currency/Doubleday, 1990.
Norman, D. The Psychology of Everyday Things. Harper-Collins, 1988.
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