If you hook an LED to a dc power pack and slowly turn the power on, you'll see that they will come on all at once and then grow brighter till they suddenly quit and are destroyed by too high a voltage. You can adjust the brightness somewhat with resistors but they are ot as "linear" as incandescent
Yes it seems because LEDs are very directional and looking into the lens it doesn't seem to change that much - but if you look at the beam shining on something it is easier to see the intesity of the light change. If you use a 1K pot and 3.6v and experiment...
Dan - I find that the best way to dim LEDs is with a pulsed DC voltage sometimes called PWM or PWC - I wrote an article about dimming and brightening ditch lights that goes into quite a bit of detail about how this can be done.
I had 2 white LEDs in series on a 14.4v battery powered locomotive and wanted them quite dim. I put a 10K resistor for current limiting. They WERE dim. A 1K resistor on 14.4v will dim them but not drastically.