Great posts on this thread. Here are some issues to consider from my current experience with a temporary layout that I have had for over a year before I tear it down for the permanent one:
1. What is your purpose? For me, I want to engage my grandchildren and have fun operating a variety of rolling stock and vintage PW accessories. Of course you must be sure they all run!
2. Available space and configuration for layout. This is a major factor.
3. Clearances. Although I have a sketch and computerized layout, best way to do this is to set up the track and run the trains. I have been modifying my trackplan about every 3 wks to accommodate another issue (clearance, accessory, scenery, etc). Each time it gets better.
4. Height of layout and any additional levels. Mine is an unconventionally low main level for grandkids. Also need to determine what you want in their reach...and what you want out of their reach.
5. How scenery will work with the layout and clearances, such as walls, tunnels, switch locations, etc.
6. Resources for backdrops, benchwork, wiring, equipment maintainance, scenery. Lots of great books, magazines, and of course, this forum.
7. Type of track. Lots of personal opinions on this. If money no object, I hear Gargraves track with Ross switches is way to go. For me, I have stayed with my my tubular O gauge track and PW switches.
8. Conventional, command control operation, or both?
Bottom line is make sure the layout and operation work for its intended use...and include fun in that!
Michael