There are a few things to remember when designing a map for QuakeDC.
Here we'll list what we know as we find it out...

Size.
One very big limit for QuakeDC is its amount of available ram. The larger (in Kilobytes) a map is the more likely it will not load or will be missing textures, etc.
A way to keep your filesize low is to limit the size and number of the textures used
Wide open areas are never a good thing in a quake engine'd game. QuakeDC seems to not handle it as well as winquake either. If well thought out you can still have somewhat large open areas. This also has to do with the next subject...
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R_SPEEDS
QuakeDC seems to be more sensitive to the number of polygons then being drawn then PCquake. (May have something to do with QuakeDC's use of Direct3D) Because of this R_SPEEDS is a very important issue. R_SPEEDS are directly related to the number of polygons drawn. If you reduce the number of polygons drawn by the engine you not only improve QuakeDC compatibility... but also improve the overall speed and performance of the engine with that map.
Here's a link to an excellent guide for reducing the number of polygons in a map.
Polygon Reduction Methods
[ by David Hyde, Mad Dog, and Richard Neff. Though this is aimed at Quake2 it is very informative. ]

Transparent Water...
QuakeDC supports this feature and, though it does make the framerate suffer a bit, it is a nice thing to have. Below is a link to a site with all the info and tools you should need to make your maps support transparent water or to update existing maps to support it.
Quake Info Pool's Transparent Water Tips
A site to get prt files for many popular maps (needed to convert them to support transparent water) is here...
WaterVIS

Worldspawn
If your building your map for use in a custom engine or for use with a custom compiler you will want to open your compiled BSP in Quark and edit the worldspawn section of its entities list. You do not want anything in there other then the standard Quake stuff. No _sunlight or _fog or anything fancy... QuakeDC does not like _sunlight and will refuse to load the map. Other custom keys are untested as of yet and may or may not work in QuakeDC... but engine specific features (colored lighting for example) will NOT work in QuakeDC. You can check and edit your .map file with a simple text editor.

DETAIL VERSUS DESIGN
This is a short editorial by Dave J on when design the overal design of a map is more important then the fine details. He uses Halflife as referance... but there are some cases of detail overpowering design in Quake maps. Since details are often harder on the engine it is extra important to find the right balance for maps designed for QuakeDC.