The thing is, though, that SP and DM maps have different goals and different requirements.
Broadly speaking, DM maps are built for speed: players need to be able to move around it quickly, so there is a limit to the amount of brush detail you can use while still having a playable map.
SP maps, on the other hand, usually aim to create an immersive atmosphere with variations in pacing. This means that things that would be out of place in a DM map – e.g. areas that take some time to traverse (think jumping puzzles) and very detailed environments – would add value to an SP map.
Having said that, there are some general rules or guidelines that are good to keep in mind for both DM and SP, such as:
- Be careful with brush details on the level of the player, as even in SP it is frustrating to get caught on fiddly bits of architecture. Instead, place your brush details where the player cannot reach it, e.g. above the player. If you must have brush details on the level of the player, use clip brushes and func_illusionaries to keep the environment free of bits to get snagged on.
(There are some exceptions to this quideline in SP, for instance in secret areas.)
- Light your map well. This usually involves:
2.1) Not using shortcuts like simply setting a global minlight value, compiling your map with ericw’s -dirtdebug flag (as I suspect you did here) or leaving everything fullbright, and calling it a day.
2.2) Using sourced lights e.g. torches and light textures with a light entity placed nearby. Basically, where there is light in your map, there should be a logical origin for that light. Unfortunately it takes some practice to make this look good: if you just add a couple of torches or light textures with accompanying light entities to your map, you are likely to end up with a map that has far too much contrast and is pitch black in places. The secret is to play around with different light values etc. until you reach the right balance between flat and overly contrasted lighting. Here I would once again recommend Func_msgboard as a resource.
Varying floor height and creating different levels within your map. Just having everything on one uniformly flat floor makes for a really boring map, be it SP or DM.
Using a variety of textures for detail. Though it is not impossible, it is hard to create something visually compelling using just three textures (especially if your brushwork is very simple), and the blue stock textures in particular become oppressively boring if there is nothing to break the monochrome monotony.
I had a quick look at your map, and it is all on the same uniformly flat level, with uniformly flat lighting and very little variation in texturing. The brushwork is also very simple. As I said, I’m not the best person to advise you on how to build a good DM map, but if you’re just using this as a means of practicing making something for Quake that looks good, then I would say try and make the brushwork a little more varied and interesting, and refer to the points above.