These are just some random tips about mapping. I do not claim to be a wise figure in the scene, so feel free to ignore my advice.
Know Your Tools
Before undertaking any serious attempts at mapping, you should understand what sort of possibilites the game engine offers. This includes playing the entire game from start to finish, and perhaps a wide assortment of custom user maps. It is also a good idea to familiarize yourself with the map editor beforehand. You should at least read through "The Basics" section and create a few (or a few hundred) experimental 'learning' maps. The "References" section also contains loads of useful information for experts and novices alike. To gain some insight on the design process, load some of the official maps in the editor and tear things apart to see how they work. In any case, do not upload your 'learning' maps and present them as if they are a completed work. This will only serve to damage your reputation, and nobody will take you seriously afterwards.
Novice Mistakes
There are quite a few mistakes that almost every new mapper seems eager to make. Just a few of these include: 1) Placing strippers for no apparent reason, 2) Massive empty areas, 3) Parallaxed skies sporadically set around the map, 4) Extremely thin or thick doors, 5) Unrealistic scaling, including cramped hallways and giant objects (use F7 in-game to get a better perspective of the scale), 6) Overusing jokes from the game verbatim (especially placing an Assault Trooper on every toilet in the map), and 7) Placing way too many enemies and/or not enough health and ammunition. These are not major issues in 'learning' maps, but none of them should appear in a properly released map.
Inspiration
It's okay to crave positive responses and recognition from other players/mappers, but don't let this be your only source of inspiration. It is simply impossible to please everyone, there will always be someone with something negative to say about your work. Build maps for you, not for other people. Your mapping experience will be much more rewarding if you don't get hung up on other people's opinions.
Ideas
Try not to use all of your ideas at once. Choose your favourite, and begin expanding on the idea until you have enough material to create a map. Choosing your strongest and most original ideas always makes for better maps, especially since they will maintain your interest and keep you focused. One of the most important things to realize is just how flexible a hobby map editing can be. There is no rule that says your maps have to be designed strictly for the world of Duke Nukem. You could just think of it as developing a mental world of your own, that also happens to be 'playable'.
Variety
There are literally thousands of ways to create doors and switches. Try something original. Remember that a door can be constructed from multiple effects, all of which can be activated at once by a single switch. Make your switches interesting, too. Remember that you can hide a switch by making it one-sided and facing the wall, which allows you to effectively morph any object into a switch. Over time it's become boring to hit the same old switches and search around trying to find out what they did. When a player hits a switch, make it exciting. Also try to avoid making maps completely flat. Add slopes where appropriate, and force the player to run, jump, and climb once in a while. In a similar vein, avoid making everything square and blocky. Add subtle shape differences to every sector. Don't leave the player stuck with one weapon for too long, either. Let them 'graduate' to a bigger and better arsenal as the map progresses, and give them more alien fodder to test it on. An original level should have loads of variety.
Shading
You would be surprised at how intense an effect shading can have on a map. Even a plain-looking, empty room can look shockingly impressive with intricate shading. When you add light sources to a map, try to gauge what their actual effect would be on the surroundings. Don't forget that areas which are devoid of light sources should be dark! Also, use coloured lights in places where they might seem appropriate.
Security Cameras
A lot of people place security cameras to show off the whole level from a single viewscreen, sometimes right at the beginning! This ruins the point of playing through the level. If you do choose to make security cameras, at least make it display something interesting or vital to the storyline. Remember you can convey messages to the player through viewscreens. Just build a room somewhere with your message and then direct a camera at the message. If you're going to show off the whole level from a single viewscreen, at least place it at the the end of the level rather than the beginning.
Touchplates
If your level is based on action, use touchplates everywhere. Make every room spring to life upon the player's entry. The player's presence should be what makes the world revolve. For example, when a player is moving cautiously through an empty hallway, make a door blow open and throw enemies out of it (maybe with a hidden conveyor belt?). If there isn't a surprise around every corner, the player will quickly lose interest.
Escape Routes
Nothing is more exciting than diving into the window of a moving vehicle or leaping across rooftops and landing inside a building. This is especially true when you're completely surrounded by attackers. Sure, fighting hordes of enemies can be fun, but narrowly escaping certain death is even better. Don't always give them a way out, though. Force the player to fight once in a while.
Problem Solving
Set up problem scenarios that make the player think about options and outcomes. Pipebombs can be activated remotely, moving sectors can carry items around (including cameras!), the player can be shrunken or given jetpacks and scubagear, explosives can be sent through teleporters, etc. Come up with different ideas to keep the player resourceful. Give them a few weapons or items and let them figure out ways around obstacles. It's also fun if there are multiple routes throughout the level, so each play will have something new to try. Avoid boring puzzles and mazes (combination switches, etc). These distract from the action and require no thought, which usually makes the player resort to "DNCLIP and run through the rest".
Curiousity
Design detailed areas beyond the boundaries of the player area. When a player sees detail in an area that is inaccessible, it creates a sense that the level extends forever in its own world, which engages their imagination and sense of curiousity. Use curiousity as a driving force: The player will try some pretty crazy stuff to get to inaccessible areas. This explorative drive can also lead them to find secret places. The DNCLIP cheat sort of ruins the fun, though.
Secrets
Adding secrets to your map gives players more incentive to replay it and explore it much more thoroughly. Secret messages are also fun to search for (as long as they're interesting). I suppose it's a matter of opinion just how well-hidden the secrets should be. Usually, you would want to give the players at least a reasonable chance of discovering them on their own, otherwise they'll just get frustrated and cheat to find them. If a player has the determination to find a well-hidden secret, reward them for their efforts.
Realism
How realistic you want your map to be is entirely up to you. Don't forget that there are fantasy elements built into the game by default (aliens, shrink rays), so realism is usually focused on the map construction itself. This includes shading, choice of textures, design and layout, etc. You could also go the opposite route and make an entire map based on abstract concepts, which has been done to great effect in the past.
Testing
The testing stage of completing a map is also one of the most time-consuming stages. Be sure to playtest your map until you are completely satisfied with it. Test it in every mode and skill level that you have implemented. Intentionally try to cheat the logic of the map in order to find errors. There should be no areas where the player could accidentally get permanently stuck and be unable to finish the level. Rearrange enemies, health, weapons, and items wherever you find it necessary. Also keep an eye out for misaligned textures (use the period . key to automatically align textures). If you're looking for an outside opinion, you may want to select a few people to beta-test your map.
Skill Levels
When you are implementing skill settings in a map, adjust your style of play accordingly. For example, while testing a map in "Piece of Cake" mode, I usually hold the controller with one hand and purposely play in a clumsy manner. While testing "Let's Rock" mode, I increase my senses somewhat and pretend that taking on a medium-strength enemy is a big deal. In "Come Get Some" mode, I just play the game as usual: Shoot fast, think fast, move fast, etc. All of these modes should provide a slight challenge against that particular style of play. Finally, if I can make it through "Damn I'm Good" mode alive, then I haven't done my job.
The Ending
An excellent ending will leave a lasting impression on the player. It doesn't necessarily have to be 'epic', but it should be as interesting as possible, and it should fit the context of the map. An ending that comes too soon or too late can ruin an otherwise great level. If a map is consistently well-designed, but drags on for far too long, consider splitting it into multiple parts or creating an episode.
Plot Development
Create a story for your levels. Players will become much more immersed in playing a level if it has a storyline backing it. Almost all Duke level designers accomplish this by including a small .TXT file with the map describing the story and the objectives. If you know how to write a batch file (.BAT), you can use it to tell the story and then load the level automatically. Learning batch files is very easy, look it up online (I think most Windows help files include some information as well).
Reinventing
Yes, Build is an ancient engine. Just remember, you can never run out of things to make. There are so many new tricks still waiting to be discovered, and so many designs and styles that have never been seen before. If you're really ambitious, you can learn how to recode the CON files, and add new art and sounds to create a whole new experience using the same game. All you really need to create a great level is time and creativity. Limitations fuel creativity.
Signature
Once you're finished your map, draw sectors somewhere off in the corner representing your nickname or initials. This is so everybody knows who slaved over that fine work of art.