why is my led bulb running dim?
Well, turns out that repeated slamming of the front entry door isn't good for many things, electrical connections included.

I finally had a chance to open the outdoor sconce yesterday and saw that one of the neutral white wires had sort of slipped out of its wirenut, and wasn't holding on very well to the others. I don't know why this resulted in lower voltage going through instead of arcing, but that's probably a good thing.

Fixing this was a simple matter of winding the tip of this loose wire a bit deeper and screwing in the wirenut a little stronger than it already was. Did this to the hot black and bare ground wires too, and the light's as good as new now.

Lesson learnt? Electrical gremlins don't need to be caused by faulty installation, sometimes things that you don't expect to break (like immobile wirenuts) also loosen up due to mechanical vibrations.

Next step? Trying to convince the family that slamming doors isn't a good idea, but that's a battle I'm not going to win in any case.