One of the real important things before you start with your map is to think about the setting. What is your map all about? Another UAC base? A Dungeon? A Spaceship? If you want a map that really gets some public attention, think about something, that hasn't been done before or something that can be done better. There are thousands of uac base/city maps out there, but just one map like Cyb's Void (shot 2) or Russel's Null Space (shot 3). When both of these talented mappers started with their maps, they thought about something new, something that has never been done before and that was the reason (beyond the fact, that they did a great mapping job on their wads) why both of these maps were so damn successful! So, if you are about to choose your setting, be creative, the only frontier you have is your imaginary, because nowadays, most of the source ports out there are capable of doing everything, even such an Armageddon effect seen in TNT4 (shot 3) And don't misunderstand me: I don't say, that yet another UAC Base can't have such a effect to the Doomers out there, but - as we are all curious and keen on sensations - many of us want to see something new, something that hasn't been done before and if you look closely at the "Annual Caco-Awards" you will see that most of the laureates are WADs with stuff that is new, something that exceeds the player's expectations! Go and convince yourself ;)
Summary: "Be Creative, Be Innovativ, Do Something New!"