Round all corners and edges. A simple and effective design principle so often missed without reason. Computer builders endured sharp edges until case manufacturers started rounding their internal edges with blunting, closed hems, and continuous powder-coating. Exceptions apply when function requires a sharp edge (ex. knife) or when rounding is cost-prohibitive. When large rounding lowers visual aesthetics, use small rounding; better some than none. Use tumbling for economical blunting of complex, durable parts.
Put a handle on it. For comfortable, convenient carrying.
Support one-handed operation. For multi-tasking convenience (ex. one hand holds product, one hand holds key to unlock door). For emergency usability (ex. other hand is injured).
Make the top flat. For a makeshift table surface. For sitting. For stackability in organization and storage. Slight angle prevents rainwater pooling, improving outdoor lifetime.
Make the bottom flat. For stability. For ease of cleaning (single surface). For even weight distribution. For resting on top of a shoe when users are trying to comfortably keep it away from a dirty floor.
Put impact-resistant feet at the bottom. To minimize contact area with the potentially dirty floor. To dampen impact vibration before it reaches the frame and internals. To wear out ablatively and be replaced more easily than the exterior bottom frame.
Make it small enough to fit inside common places. For holding in hand. For keeping in pocket, handbag, or backpack. For resting on the seat or trunk of a car. For storage inside standard size envelopes and boxes.
Make it small enough to move through common gateways and passages. For passing residential doors, commercial garage doors, hallways, elevators. Make inner spaces large enough for common handling and observation. For fitting a hand to clean inner surfaces with a towel.
Make it light enough for average people (or most people) to hold with one or two hands. Weight is a key reason why cast iron pans are not as popular as they could be.