Aesthetically, I like it when the spawns look like they have a job, the're meant to be there as part of the story the map is telling. Some spawn rooms look like they were obviously just just 4 walls, a ceiling, and a floor tossed in there for the need of a spawn and then, maybe if there lucky, get detailed.
I also like when the details inside the spawn tell a story themselves. For example, Upward's spawn with the gondolas (i guess you'd call them) bringing up the bombs from the blu cargo ship down below -- how else are you gona get a war head up a steep cliff face without anyone noticing? A more subtle example is, like what I said before, the resupply locker on the hand truck in Frontier. Obviously that little cave is just a dump for some unwanted dirt dropped through a hole in the ceiling. But blu decided to wheel a resupply locker in to it and use it as a spawn room. This goes along towards many other ideas, the ambulances, the air boat in Mt. Lab, the blu box car in junction dropping off troops, and many more.
This obviously has nothing to do with game play, but its just these nifty little gimmicks that make me laugh and appreciate the gumption of the mappers who come up with them.