Today the topic is the relation between utility, usability, and beauty. Tools need to balance these three drives.
Let me break these out a little.
Utility is the function of tool to let the user do their work. A useful tool is one that increases the capabilities of the user, either by magnifying their existing abilities, or providing a new ability that didn’t exist before. A nail hammer provides utility by allowing the user to insert slivers of metal into wood; the user could insert the slivers other ways, but couldn’t insert such large slivers, nor so quickly, nor at such a low cost of effort.
Usability is the function of tool to reduce the overhead for users to do their work. All tools incur overhead, work that you didn’t need to do before, but need to do because of the tool. (This is sometimes called “tool work”.) A usable tool will provide enough utility that the improvement in task work outweighs the increase in tool work. Note that this doesn’t necessarily mean the tool has high usability, just that its utility outweighs that lack. Hammers are eminently usable; the handle affords grasping, the head position promotes good nail contact; but this is the result of centuries of refinement.
Beauty is the power of the tool to engage the user, and is fundamentally connected to both utility and usability. A beautiful tool uses appearance to help the user build a mental model that matches the actual function of the tool. Beauty doesn’t always mean simple, though it sometimes does. Google’s trim appearance is beautiful for the complexity it hides. Victorian homes, with their baroque ornamentation, can be beautiful for the craftsmanship they showcase. Beauty-as-elegance is familiar to most tinkerers (hackers, designers, etc.) — elegant design of products. Beauty truly derives from the connection between appearance and function. A beautiful hammer communicates its use through shape and balanced weight; an ugly hammer distracts the user from the task, possibly by cramming too many features into one tool or poor choice of materials.
The process of design allows creation a tool that balances these (sometimes opposing) three drives. Utility drives a tool’s design; usability channels it; beauty refines it, allowing space in the user’s understanding for more utility.
The best tools are useful, usable, and beautiful: they have beautility.
 I use the word ‘tool’ after the tradition of Activity Theory. A tool is any object and/or process that people use to get a task done. A hammer is a tool. So is Firefox. So is a grocery list. So is the ‘script’ you use when ordering food at a restaurant. So are democracy, a music score, and the process of a baseball game.
 ‘Appearance’ is also an overloaded term; I use it akin to how some people use “look and feel”.