Tips for creating good WADs

Things
If you intend to create a good, basic Doom level you should at the very least include some monsters, weapons, ammo and health pickups in it. How you place them is especially important. Some general tips for good item placement:
 * If your level includes powerful monsters, it should also offer powerful weapons. Very few people want to fight a Cyberdemon with a pistol.
 * Do not give out the most powerful weapons right in the beginning. Giving the player a BFG and a huge bunch of cells right away is rarely a good idea, since it tends to lessen the suspense of the level by making the player feel all-powerful and the monsters much less of a threat.
 * Items (especially the best powerups) should be used sparingly, except for ammo.

When placing monsters, it should be kept in mind that well-received levels and levelsets usually get progressively harder toward the end. Placing ten Spider Masterminds in the first room of the first map is generally considered bad taste and too over the top design. Instead try to create balanced and imaginative fights that do not repeat themselves. This can be achieved by using different monster sets, varying level architecture and by giving the player only certain weapons, ammo and powerups.

Architecture
Most people value gameplay over aesthetics, which means the structure, layout and thing placement of the level is more important than the way it looks. It is often a good idea to visualize the basic layout of the level before actually building it in a level editor. After the layout is ready, you can build the actual map and add the details. Adding a huge amount of detailed architecture is not necessary (excessively detailed architecture can even annoy people), but few people like, for example, square and flat levels ala Wolfenstein 3D either. In the end it does not really matter whether you use very detailed architecture or a more simplistic approach, as long as the level looks good.