A point that trips me up on so many diagrams is that they don't seem to have a direction to be read in. Either left to right, top to bottom, or reversed. Sometimes it can make sense to have a loop, so top is left to right, and bottom is right to left. But in all cases, have a reason for something to be where it is.

Of course, then there are the odd "maps" that people love to have to show isolated concerns, but there is no real meaning on how the map is traversed. I find these neat artistically, but they don't actually communicate that much to me.