My 2 maps that I have made so far (cp_desertfortress and ctf_convoy[unreleased]) started at 2 completely different angles.
With desertfortress, I started with a highly thought out basic map layout and design on paper, and then struggled trying to wrap a visual theme around the map design. The result was a map that doesn't have much visual quality or purpose to it's architecture, but plays extremely well and is very balanced. The few clan server admins that I tested out the map with strongly agree even though it could still use a few tweaks. The trade off was that is not that popular because it doesn't have the visuals to make it stick out or sell really well like other maps, for example highly played maps like castle which have visuals that stand out.
With convoy, I started with a visual idea and theme (2 rocket carriers racing across the desert in a constantly moving map) and started trying to fit a map design around the visual theme. My prediction is that this map might sell allot better than desertfortress because of its visual theme that sticks out. The gameplay might not be as intense, but it might end up being played allot more.
There is really a hand in hand trade off between going with strong visuals, or a well though out layout. We all have a finite amount of time, and a limited amount of talent so we tend to distribute our efforts biased in one of those directions. I spent months working on the map layout for desert fortress on paper, long long before I even touched hammer, so much less of my limited effort went into the actual building of the map.
