Functional products simply do what they are supposed to do.
Safe tools shield danger, emit warnings, fail small before failing large, and sacrifice themselves to protect the user.
In truss construction, steel is preferred over concrete because steel provides reliable strength over a greater yield curve. See Florida International University Pedestrian Bridge Collapse | Forum Discussion at Eng-Tips.
In fitness equipment design, aluminum-bronze bearings are absolutely used over lead-bronze bearings because lead is a highly toxic heavy metal.
Reliable products continue to work after prolonged use in extreme applications. Reliable designs generally use simple mechanisms with long-lasting materials. Vulnerable parts are shielded, such as with plastic encapsulation for integrated circuits. External parts are fixed in place, such as with split pins for industrial bolts. Reliable products run through simulations and real use experiences to pass all pre-defined requirements and common-sense expectations. Detailed tests bring confidence that reality will and does actually conform to the intentions of the design.
Durable products resist breaking under extreme environmental conditions. Durable designs avoid or strengthen all weak points, such as upgrading plastic parts to metal ones. Some notable durability considerations include galvanic corrosion between dissimilar metals, seals exposed to harsh conditions, and vibrational effects on machine joints.
Convenient products are easy to use, hard to lose, and readily accessible in times of need. Aesthetically intuitive designs help first-time and returning users operate the product properly. One-piece and unified designs prevent the separation and loss of functional parts, such as with cars connected by straps to their gas caps. Low maintenance designs help the user keep the product in likely working condition.
Affordable products are inexpensive enough for the average customer in the market. Price barriers determine whether a target demographic can justify buying a product of even outstanding quality. A notable price barrier separates the market for individuals from the market for businesses. Fundamental innovations and design tradeoffs improve cost factors most effectively; consider the economical production of membrane keyboards, which are less performant yet markedly cheaper than their mechanical counterparts.
Versatile products solve problems in a variety of applications. Good versatile designs seek to mirror the reliability of its single-use substitutes as much as possible, sacrificing price and ease of maintenance instead.
Small products save space and fit in more places. Even when the product's function requires a large size, such as with a large dinner table, consider how to reduce the size of non-functional components, such as the table legs. Consumer products must be small enough to fit through a typical doorframe. Industrial products should be small enough to fit within a typical cargo trailer or box truck.
Light-weight products are easier to lift, hold, and operate. Light products are more practical to carry in pockets and backpacks; more safe to store in high and sloped locations. Light designs are more compatible as a module of a larger system. For instance, a camera-on-tripod assembly is easier to move across a filming site if the camera itself is light to start with; a robotic arm with a light end-effector has a higher maximum payload.
Quiet products can be used continuously without causing a distraction. Quiet designs are particularly important for applications requiring close human presence, such as with residential appliances. Other times, noise reduction is necessary for sensors to detect signals from a relevant source, such as with an airplane air leak. A quiet toolbox is designed with a quiet-impact material either built as its frame or padded on its closure contact area.
White surfaces are markable with high contrast.