The two differences are 

 - Cost 
 - It can be dimmed with a standard dimmer

A standard dimmer reduces the RMS voltage going to a light to dim it; this may be done by a simple resistor or by chopping of the top of the waveform.  So a dimmable LED lamp must be able to:

- Cope with a big range of into voltage including part formed sign waves.  (Not easy for the control logic)
- Detect from the input voltage the light level that the user wants, then give that light level.

It would be a lot better if a dimmer switch could send a digital signal (radio, or over the main wiring) to the lamp, so that the lamp could know what was required without reverse engineering the voltage back to the user's wishes.   However this would need a new standard to be used by all switch and lamp vendors, so may take a little time-:)