In sequence:

Decide where you want your zones for best flow, i.e. hot, cold, dry, wet, OR cooking, food storage, prep, serving, cleanup.

Choose appliances and place into your zones.

Decide on storage requirements. Choose cabinets & configuration; adjust dimensions & appliance placement.

Then check for possible interference.

My #1 would be relation to and placement within the home and land first. For instance, for someone who loved a sunny room, no amount of cabinetry would make up for lack of windows, and those windows facing the right directions. A great kitchen is a space that draws one in.

gizmonike and rosie, I was being rather limited in my thinking, more about when you're looking at an existing plan - but those are great points for getting to the stage of having a plan at all :)

Keep 'em coming folks

Kevin

My #1 is lighting - both natural and artificial. Where are the windows (yes, plural) located and are the light fixtures situated in places that will adequately light the work areas.

this is adding to, not voting on, right?

Venting -- can it be done where you might be thinking you want it?
Dishwasher: right or left of sink?
Blocking sights: so dirty dishes/cooking areas aren't out in the open if kitchen is an open-type.
Safety: where is the oven opening and where is the stove located?
Seating: part of the aisle space issue.
Cooking: space on both sides of the stove.
Landing: also for the oven.

Here's one that maybe people address by installing prep sinks but I have no interest in a prep sink: the line between the wet, chopped up vegetables and the cooktop/range. Does it require dripping and dropping the stuff all over the floor (or in one layout that was just posted, carrying it across the entire kitchen) to get from the sink and cutting board to the pan?
Double ovens: what does that do to the general view of the kitchen? Does it block light?
Refrigerator: where can you recess it?

For me, the starting point was: windows, sink location, double oven location, and sink/cooktop relationship.

great points Fern, thank you!