I used to make Quake 2 and AvP2 maps, and the primary limiter of my abilities was my crap computer, with the secondary limiter being the tools. I used Qoole for Q2, but its editor windows couldn't display very many polygons, even in wireframe editing mode. I had to make levels in chunks, then tape them together and hope everything aligned properly with no opens. DEdit for AvP2 was its own sort of fucking mess, and would regularly fuck off in random ways when compiling.
Anyway, I think the primary skill for mapping would be a strong imagination. The ability to visualize the planned map in its entirety, as if it was a real place, then use the editor to make a "simplified" version of that reality. The reason why many people cannot do this is because they lack the mental facilities to form virtual worlds. A strong imagination requires a lot of mental processing power, which translates to high intelligence. I think of imagination as virtualization, like making a sandbox computer within a computer, your imagination is used to create virtual worlds within the space of your skull. Those who are incapable of this are typically idiots. Look at who bothers to make signage, products, and miscellaneous infrastructure items for their game maps. Amateur as my efforts were, I still put goddamn street lights, curbs, manhole covers, power lines, pop machines, and dumpsters in my city maps.
People seem to be unable to focus even on the broad basics. What is the "gimmick" of the map? How will this map be fun for players? What does it bring to the table that others do not? Are there secret paths? Traps? Once the broad strokes are laid out, more complex details and fluff can be done, like the shit I mentioned above. It's a shame that stuff like Quake, Doom, UT, AvP, and Battlefield are now limited to the most diehard of fans, and big communities like Polycount are shadows of their former selves. The early 2000s were a treasure trove of custom maps, player models, skins, mods, and total conversions. Shit, Battlefield 2 came from the Desert Combat mod for Battlefield 1942.