On thing that is heavily underestimated - although it is that easy to achieve with DoomBuilder - is the creation of borders, whether in rooms or outside your buildings.
Many texturesets - the doom one included - are very hard to combinate to develope new mapping styles. Often, their edges, where ceiling/floor and wall connect, look damn ugly.

In the first shot, we see a combination of TEKWALL and this E1 metal floor. Everyone knows how terrible this would look like without the metallic brown border. On the other hand (as it was done in E1M3 of the original Doom), another border has been added to the top of this TEKWALL construction, where we can see the red TLITE flat. This border makes also the crossover from one to another texture much more professional and visually more admireable. (buildtime: 2-3 extra minutes)

In the second shot we can see another place where I heavily used this "Bordering" method to develope easily a high level of detail: A border was added to the left, where CEIL5_1 and the bright brown wall (BROWN1) meet, another border was added to the top where you can see the sky window, easy to achieve, but the effect looks amazing. (buildtime 6 extra minutes)

The third shot finally shows another alternative of making connections between different materials smoother, the "irregular bordering". Let's say you have a artificial floor and want it to connect in any way to something more natural. Well, just make it look as it is destroyed in some way. Just take a look at the screen and you will know what I mean. The good thing about it: It looks damn cute, although the floor is maybe not supposed to be destroyed but who cares? It's a science-fiction game and no-one will ever say/ask: "Why the f*ck was this floor destroyed?!"

Screenshot number 4 shows us another tip concerning borders, especially those in outside areas with some kind of natural origin. Although this BROWN96 texture shouldn't have this border at it's bottom (it's a rock, why should rocks have a border?!), I added it and you have to admit, it looks better then just leaving the brown rock as it is in E1M1 or E1M8! This is just here to prove, that you can add borders everywhere and truly they will always look better after this medicin ;)

Summary: You can use this method of adding detail almost to ever room or place you work on, it is fast, the result looks great and it isn't that hard to create. 
Extra-Buildtime: "2-10 minutes" (depends on room-size)
Build-Difficulty: "Low"