"...which is that a regular diode has no trouble living in the current stream of my wiring, but an LED does..."

A regular diode is built to handle a certain current in the forward direction -- say, 5 amps. If you impose a load that draws, say, 15 amps -- pop goes the diode. Usually, the junction overheats and melts and it conducts in both directions. Hence, the diode is "shorted."

An LED needs something around 25-50 milliamps (or about 0.05 amps) to light. If you put it in a situation where it passes more than, say, 100 or 150 milliamps, it overheats, melts, burns up. The resistor limits current to whatever is appropriate for the specific LED.

"Does an LED also limit current to one direction like a reg-lur diode?"

Yep. The two-color LEDs are actually two LEDs connected in parallel but with opposite directions. Thus, current in one direction produces red, current in the other direction produces green, and AC (which is alternating current) lights both junctions, which produces red AND green, or a yellowish orange (color depending on relative amount of red vs. green).

"And what makes them so vulnerable to current?"

It's the way of life. Magic. Karma. Zen. Harmonic convergence. Junction resistance, physical size, and heat dissipation capability.

I like your thinking about LEDs replacing many incandescent applications. Already we can buy flashlights using white LEDs. Obviously, automotive and truck taillights and directional flashers are switching from incandescent to LEDs. Stoplights at traffic intersections are using red LEDs. As laser and LED technology develop and overlap, I'd expect to see some really, really bright white light (actually, three simultaneous colors -- RGB) emitters. Maybe cool, efficient lamps for home applications?

Once the brightness problem is solved, why not headlamps?
--
Don Foster (near Cape Cod, MA)