Watts, lumens, Kelvin, CRI, IP, and other useful terms.
Amount of light emitted: more lumens = brighter.
Power consumption; with LEDs, lumens per watt are more important.
Color temperature of light.
Color rendering: how natural the colors appear (ideally >90).
Dust and water protection (e.g., IP44 for the bathroom).
Compatibility with dimmers; check the driver and lamp.