Prof. Steven Postrel has a good post over at Organization and Markets about why some entrepreneurial opportunities go unsatisfied. His example is why outdoor coffee tables have four legs instead of three, when it's "obvious" that three legs would make the table rock less. He concludes with the following design puzzles, which reminded me of the book Why Not?.
“Pick a horse” checkout lines (e.g. in supermarkets), instead of standing in a common queue and getting the next available clerk.
Multi-piece towel racks, where the bar can slip out of the mounting brackets if they loosen up and flex a bit over time, instead of single-piece racks where the bar is permanently attached to the brackets.
Standard spring-loaded paper napkin dispensers, which invite overloading to the extent that the first five or six draws produce only shreds, and which are occasionally loaded with the napkins oriented so that there is nothing to grab onto. Alternatives include the vertical one-at-a-time dispenser recently deployed at some McDonald’s.
Auto-numbering in Microsoft Word, which behaves like a peevish poltergeist, randomly changing number and letter headings, creating and destroying tabs, etc., instead of almost any other numbering utility I can imagine.
Medical bills and insurance forms, which go out of their way to obscure what was charged, what was allowed to be charged, what was paid, what you owe, etc., instead of almost any other reasonable design.
Mustard packets, which for some reason are much harder to tear open cleanly (the tear tends to go too deep) than ketchup packets.
CD jewel boxes, which are hard to open, expensive to make, and fragile, instead of cardboard sleeves sealed in plastic wrap.
(Hat tip: Virginia Postrel)