LEds are the light of the future, and work wery well in many applications, such as traffic signals, where they are nice and bright and use only a third of the power, but, high output in a "bulb" of the size of a halogen or HID is not yet possible, thus making reflector and lens design different and the whole system much larger. LEDs don't take much power, but they are not 100% efficient either so what doesn't get turned into light creates as heat and must be carried away by some sort of "heat sink". Glass, quartz, and tungsten can run at very high temps, the materials in an LED cannot, which limits the the amount of light which can be obtained from a unit the size of, say, an H3 55 watt halogen, a very common bulb in fog and driving lights to probably 20 watts. In two to five cell flashlight applications (3 to 7.5 volts) there are LEDs which will equal or exceed the output of standard filament bulbs, and can be very bright, but beyond that the heat sink requirements cause the size of the LED and sink to increase to the point where it won't be a drop in replacement or use the same reflector design.
LEDs are single color (unlike a filament) and the most efficient color is BLUE right now. To make a white you need the three primary colors in one unit, (big) or to take advantage of the blue LED higher output per watt a coating is applied to the blue LED lens which reradiates white light- though some of the blue leaks through and gives that neon effect.
The best small, high output units today come from a company called "Cree" which makes some pretty good looking white elements
as well as single color units which can and do make good turn signals for cars and bikes with low power consumption and standard sizes and sockets.
