I often find I dislike the colors used in heat maps. The designer (often not an information designer) assumes that red is the most attention-getting color, and that throwing a lot of different rainbow hues of equal brightness is the way to go.
I think yellow attracts more attention than red does in these diagrams, and brighter values call the eye more than darks do.
I see Edward Tufte agrees with me, so you know he's gotta be right. ;)