I think that this thread will fit somewhere between the "What did you do on your layout today" and "Is the desire to expand ever satisfied?" threads. I hope that it will provide some food for thought for those building new layouts.
Warrenville started with O27 curves, a year or two later it was clear that was a mistake. Thank God that the time Warrenville was only about a quarter of the size it is now. The destruction to change all of the curves to O42 and O54, and switches to O42, was great. *Start out with the widest curves that you can fit.
In the beginning I was accessory and structure crazy - buying whatever I could and squeezing them in wherever I could. Decades later I'm still removing items and moving others to more logical places (See my post yesterday in the What did you do on your layout today?" thread as an example). *Take a deep breath and plan your industries and towns, in a logical manner, from the beginning.
Once scenery is done, ya just cant just walk on the layout for access to center spaces. *Plan access between the layout and walls and/or access hatches when designing.
I'm guessing that you guys/gals have more layout designing oopses to share