DooM editing is an incredible outlet for creative energy, limited only by your imagination. Here are a few things I've learned along the way; perhaps they'll be of some use to you.
1. Start small, but stay ambitious. Think of your first few levels as a training ground, where you learn the basics and become familiar with the tools. Beginners sometimes get carried away and bite off more than they can chew -- going for a 32-level megawad TC with new graphics, sounds, music, etc. is best left for a team, or for someone that's been around the editing block a few times.
2. Write down your ideas and draw out sections (or all) of your map. This will give you a sense of your scale. Avoid large areas or rooms, as they often give rise to visplane overflow (VPO) errors, which cause DooM to crash. Also, large areas that are plain tend to look ordinary and boring. Detailing large areas takes a lot of work, and can often lead to visplane errors when a source port is not used.
3. Add angles, recesses, and beams to your rooms so that they'll be visually interesting. Playing in square or rectangular rooms is all right, but adding these variations makes for a nicer looking map.
4. Incorporate height variations. Have a section of a map that is one or two levels up from the main level, and a section that is one or two levels down. Elevators and stairways make for interesting gameplay while providing a nice look and feel to a map.
5. Incorporate lighting variations. For outdoor areas in daylight and reasonably well-lit indoor areas use a light value of 160 and up, for indoor areas not under direct light use 128 to 144, for unlit corridors, etc. use light value of 96 to 128. Avoid excessive use of very dark areas, as it gets old after a while.
6. Use a balanced approach to gameplay. Always provide the player with a fair chance at finishing the level or an area. Sometimes you may want to make a particular area very tough, forcing a player to find an alternative way in or to get better weapons, etc. But make sure that the player has a way of beating the area, one way or another.
7. Pay attention to the general look of the level. Define a theme and generally stick to it. Try to use texture variations to provide visual relief to your areas. For example, use STARGR1 as the main texture in a room, but occasionally insert a STARGR2. If you have a plain wall that's more than 256 units long, add a wall-bracket or "support" that uses a different and contrasting texture -- e.g., STARGR1 as wall texture, METAL1 as support or pillar texture.
8. If it's consistent with your theme and level design, incorporate outdoor areas into your map. A good mix of indoor and outdoor areas lends appeal to a level.