I think maps with enough specifics to be used at the scale you are playing would be the most obvious answer. For instance I remember traveling in an RPG on an overland map where the hexs defined an area of 30 miles edge to edge or something. Each hex detailed an encounter table for the area, the terrain type the biome and weather etc. We spent days or weeks dealing with the damn logistics and traveling thanks to an over enthusiastic player amongst us, but the point is the map served its function. Party tactics obviously dropped us down into 5 ft squares which are by far probably the most common maps, encounters were just erasable pen on a clear plastic grid and the dungeons are pre-drawn and hidden for the DM.

Map packs would undoubtedly be able to utilized most readily by RPG's because of their flexibility and the fact that they are designed for that sort of thing. For other games the map packs would have to be specific to each game I'd imagine (and most likely balanced toward the game as well).In short what makes a great map for gaming is a map that contains all the relevant information for playing that game on that map, which seems like a trite answer but also seems to me to be an unavoidable one for so general a question.