Download time is the .rsc file. Use .png/.dmi rather than .bmp. Use .ogg or one of the .mods instead of .wav. You can use preload_rsc and keep the .rsc file on a fast webserver and the players will download the resources from that instead. Other than that, every sound or picture your game includes will add to the download time.
It's not really about much imagination or brainpower, I'd say it's more about preference and whether you can bear to stare at a wall of text as a map and still feel like playing a game. Then there's remembering what each letter means.