I have never drawn a map but after a few months of playing this game I have a few ideas of what to put and not to put in a map to make it more playable.
Lag - is critical. Whatever causes it I would avoid as much as possible. Sound effects seem to slow fps. Maps with no sound like the Fort are popular.
Levels - basements or tunnels, and rooftops give a three dimensional aspect to a map. Dry Gulch is perfect example.
Lighting - big problem for me. My eyes do not adjust quickly from dark to light, neither do they pick up low contrast objects well. Maps with few deep shadows, decent contrast. Muddled or dark maps like Coyote Bluff are to be avoided in my opinion. Well lit maps (but please no harsh sun) are more playable
Size - I would assume the larger a map the more it lags. Regardless you can get lost on some maps. Think Backwater.
Spawn points - critical to gameplay in DM is where do you start? Too far from the action, takes forever to find anybody. Also don't like being dropped out in the open, that promotes spawn killing. Tillian is bad for that but hard to avoid, it is so open. An example of a bad spawn point is in the Mine, where one is dropped below the windmill across from the row of buildings (usually full of campers like me). This is going to be a BR map, but a BR map can be very playable for DM if designed well. I like BR Mexico for example.
I am sure you know all of this already, but I needed a break from playing SG.