Great topic.
I have a door-sized layout, basically a modified oval with hidden staging. A variation/expansion on the MR Carolina Central project layout. On a small layout, I find the staging one of the most important elements--it makes it seem like the trains actually go/come from some where. Other key items:
DCC
A "major" or "key" industry and a few minor ones.
A small yard for sorting.
Runaround tracks (and they're never long enough!)"
The most important element, however, for keeping it "enjoyable" is "keep it simple." My original plan had too many industries for my yard/runaround tracks/layout to handle. Too many cars, not enough room. To fix this, I changed an industry that normally would have received pellet cars and boxcars, requiring sorting in the yard, to a simple coal mine that is serviced by a separate train. I also re-arranged things to keep the cuts of cars a reasonable length and use the "grand shove, grand pull" method to get cars in to and out of my chem plant (I do "internal switching" as a separate activity). Makes operating sessions fun instead of tedious and frustrating.