basically .....anything !
The number of lights ( for each light, shadows and interference with other lights on an object has to be calculated ), the light properties ( if you use area lights, you'll be drinking a lot of coffee :p ), the number of polygons in your model ( each polygon needs to get it's lighting, reflection,etc... calculated ), the number of models in the scene, effects like depth of field, skytracer and such are also timekillers, the size of your rendering offcourse ( naturally, rendering 800 by 600 pixels is less time consuming than the 14000 by 7100 I did last week
)
so, basically anything...