I'm currently designing a new layout, and it's got me thinking about how we approach operations. Most of us fall on a continuum somewhere between the "puzzle" operator and the "spectacle" operator.
The "puzzle" operator is interested in realism. He/she likes waybills and timetables, and prefers trains to operate close to prototype, paying rigorous attention to the questions of trains' purposes on the layout. This type of operator will be attracted to big yards and/or lots of industries.
On the other hand, the "spectacle" operator simply likes to watch the trains run. He/she is probably content to spend a lot of time just running their consists around the mainline, with occasional stops to drill an industry. Good scenery and good mainline runs are the order of the day for this type of operator, who can probably get away with smaller yards and fewer industry spurs (and also, possibly, an overall smaller layout size).
If you're in the process of designing a layout, it behooves you to ask yourself what kind of operator you are, and what you enjoy most about running trains.