Is this part of the garden shady or sunny?
Sightlines:- Am I blocking or enhancing a view?
How big is the mature size of this plant? (Previous owners had planted trees too close to the house & under powerlines)
What kind of soil do I have?
Does this part of the plan look coherent?
How can I define boundaries? (e.g. hedge, fence etc.)

Ian

What is the "usage" of this part of my garden? Is it for looking at, is it for sitting in, is it for entertaining in, is it for vegetables, is it for . . . etc?

Shauna

For me:
-pet friendliness (darn dogs wreck everything)
-multi-season interest (not just tulips, not just asters...)
-versatility (if I change my mind about it, is it easy to alter)
-cost(!!)
-accessibility for watering, weeding, etc.
-what plants to do you want to grow and what will be the best location for them.

What things are there that I want to keep? A particular view, nice tree or shrub, etc. What do I want to do in my garden? How does the location/orienation of my house affect my plans?

How is the soil?
Does it rain enough or do I have to water? Blech.
What is my vision for this space, long-range?
How much time can I devote to this? Now and in the future.
Will I eventually get fed-up, pull-up the flowers and plant a forest?

I do ask my self these quesitons, all the time!