The type of light

Becuase they use different methods to create light, incandesent and CFLs produce very different types of light. The CFLs are actually producing a light that is flashing on and off (50 times a second), this can sometimes be seen when these bulbs start to flicker.

Many of these bulbs will not work with dimmer switches which can be a real problem for some people. The light given off by a CFL bulb tends to be a bit whiter and does not have the same warmth as a incandesent bulb.