1) What type of LEDs are being used and how many?
Find out forward voltage, recommended driving current, etc.
2) Do you need a constant current LED driver or a constant voltage LED driver?
3) What type of power will be used?
Running from AC? DC? batteries,etc?
4) What are the space limitations?
Working in a tight area? Not a lot of voltage to work with?
5) What are the main goals of the application?
Size, cost, efficiency, performance, etc.
6) Any special features needed?
Dimming, pulsing, microprocessor control, etc.