The lamp base / holder screw fixing position.
The physical dimension of the LED lamp and how it fits into the existing housing.
The electrical characteristics of LEDs compared to the existing system. (mains voltage, low voltage, control methods).
The location and size of the light emitting surface in relation to the luminaire reflector and in comparison to the original light source.
The light distribution, lumen output and other photometric properties like color temperature in comparison to the original light source.
The heat generated by the LED during operation and the maximum operating temperature.