Most of us are going to model a fictitious station or location. It can be based on somewhere, but the compromises needed on what space we have to model often sees us choose to make somewhere up that allows us to model with the space that we have. That I think is the main reason why more real places are not done. For example, my next layout of Briganton, which is construction is heavily influenced by a large NER style station, akin to Darlington or Stockton - but the space needed to model either would be massive or that the locations would not include some of the activity that I would like to include.
That's why Peters list above makes a lot of sense. What we all try to do is to achieve a lot of the objectives above on all of these layouts. We all choose somewhere with a place - put it in an area that matches our interests - and a time, that again is giving us the chance to run the trains we like. The place and time sees rolling stock need to match these objectives although there are some twists when you skew history using a similar example or quote a rare example that happened elsewhere that few others knew about. The twist that I enjoy is fictitious engines done up to match and blend in with the historically accurate stock and for those that notice them this again can be fun.
What Peter then moves onto are things to do with operation and running the layout, where the track-plan and design need to have provided to allow the sense of everyday and good operation to proceed. You need to have stock mirroring what a similar real location would be like, even if you then add some interesting trains to add variety. Yes there might be charters, but test trains and inspection saloons also make that list. Fact is that even with everyday movements you need to add more interest, especially when exhibiting and these can give the viewer something more to wait for or watch. Part of the fun of watching the front when operating is to see some people taking interest in the fiddle yard and then wait expectantly for an engine to arrive - before providing it for their enjoyment. It also adds something to fit these into a railway like manner, so that there is a purpose for each movement and not just a tail-chaser style operation where the same train goes round over and over and over again. That just destroys the atmosphere and creativity that a good well built layout can provide.
Which brings us nicely to that eye for the prototype - the extra small details on the layout, such as display screens, shop fronts, the cameos of people standing at locations. Its also now the extra dimension that sound can provide. Engines starting and revving to be ready for service, then moving off. Lights being used for shunting, running and standing. The extra sense of realism that a eye for the prototype can bring to model form and allow the ultimate goal of your fictitious layout being seen as a possible real place that you believe is out there somewhere...