Certainly not a complete list, but a few absolute basics immediately come to mind:
- Never apply more than 3.3V to an ESP GPIO unless the datasheet explicitly says otherwise.
- Don't drive motors, relays, solenoids or other inductive loads directly from GPIO pins.
- Use flyback diodes where required.
- Make sure all devices share a common GND.
- Don't assume the 3.3V pin can power everything you connect.
- Be careful with breadboards and jumper wires. Bad connections often look like software or ESP problems.
- Watch the power supply. WiFi transmission can cause surprisingly high current peaks.
- Be aware of ESD. Modern MCUs are far less forgiving than many beginners think.
The general mindset I would recommend is:
An ESP is basically a small 3.3V computer. Its GPIO pins are signal pins, not power outputs!