the amount of lumens it gives off and what spectrum are the most important, but you definitely don't want to have a light that gives off a lot of heat, it'll cause stress in the plants...for example, an incandescent bulb puts off too much heat, causes the plants to droop, and doesn't have the right wavelength, more than one problem at a time...

flourescents don't give off that much heat, so you can keep them pretty close to the plants to maximize the amount of light that reaches them...

from my understanding, i think the mh and hps lights may put off a little more heat, but that's just because there's more power in them, and the light is a lot brighter than flourescents, so they don't have to be so close to the plants, and their heat in turn won't fry the plants...