I know double-posting is frowned upon, but I'd rather post my ideas for a spaceport in a reply, and leave the first post for the "rules" such as they are...

Anyways, some ideas to get us started:

• HOLY CRAP THE CITADEL IN MASS EFFECT IS AWESOME:
  
  
  
• What's a spaceport without lots of different types of space ships? I mean, I've seen many spaceSTATION tilesets, but a spacePORT should be filled with many different ships docked all over the place! Different colors and shapes and sizes, possibly interior and exterior!
  
• Airports have lots of components when you think about it, here are a few that I can think of off the top of my head: Baggage claim (and carousels), Security, Restaurants, Souvenir shops, Restrooms, Waiting areas (with people waiting around), Ticket counters, Runways (and planes; see my last point), Control tower... and some of the bigger ones have trams/shuttles to get people from concourse to concourse.

Um, that's all I got at the moment, can't wait to see what the rest of you come up with.