They say that Math is the universal language; no matter what language you speak math is the same everywhere. In a way, rpg maps are also universal. There is a common language with rpg maps (maps) wherein the symbols used are used the same from rpg company to company. Even when there is some deviance, such changes are generally small and understandable.
|Map blatantly stolen from cartographersguild.com (Turgenev)|
The above map can be easily read by anyone that has done any sort of rpg gaming. Doors, secret doors, rooms, corridors, traps, stairs, pillars, statues are all in there and understandable. For example, look at room #6 and then give a verbal description of it. Likely, this description, solely based on the map, will be the same regardless of which gamer is giving the description. This common ground is a strength and something that subtly ties the gaming community together.
However, sometimes a rpg company wants to break the mold, be mavericks that set a new trend. TFT went with another approach with their map symbols. First they used hexes for their maps instead of squares. This decision was likely based off the fact their Melee combat rules, which pre-dated TFT, was hex based for better simulation of movement. This is not necessarily a bad thing though it does make buildings awkward in design with no straight lines.
Where their mapping became radically divergent was their use of symbols. They created completely new symbols for the most basic dungeon features. They also added in such things as various shading and symbols to signify different levels and elevations. Here is a sample dungeon from the In The Labyrinth book...
I'll be honest and say that I have no idea what this map is representing. They broke from the conventions of rpg mapping and it did not work out. It might be wise for any aspiring publishing company to keep to the traditions of mapping.