Most aspects have been mentioned, except the use of filters. For landscape I use a set of ND grads, and I want to pick the right one. Also if I shoot BW I might use BW filters (yellow/green/orange/red). Or I might want to use a pola filter and set it the best way possible. I want to put the focus plane right where I want it, and I want to put exposure where I want it. In the whole process there is about a million possible fuck-up combinations, so I have found that rushing it is rarely a good idea. There is so many little ways of fine tuning your setup for exposure, and I just like to try to make the absolute best of it. And whoops, there went 20 minutes. But they were good minutes :-)
See, that's something i have a hard time wrapping my head around. 20 minutes? What are you doing all that time?