In addition to the luminosity Mex mentioned, there's what I'll call Directionality, or Focus.
By that I mean that an incandescent bulb's filament shines 360* but LED's are directional. The lamp manufacturer still builds their reflector and lens to capture the light off the filament in the very best position. But sticking an LED in the socket for an incandescent is quite likely to waste a lot of the emitted light.
Our Class C had the Bargman 84/85/86 lights. Two Red Tail/Stop/Turn lenses with a clear Backup lens between. I bought complete Bargman replacement assemblies. Did away with the Bulbs AND the Socket Problems I was having. Not cheap, but they pop your eyes out on Stop and Turn.
