Well, there are attractive maps and functional maps, so what makes a map "good" really depends on what purpose the map is serving. Suppose for instance we are talking about a city map for a novel. It can be a very artistic map that shows the entire city, but if the story takes place in only a small part of the city then it might not be the best map for the novel. Another example might be The Land from the Thomas Covenant series. It isn't the most attractive map, but it offers all the details needed for the story. Jullian May's maps for the Pliocene Exiles series is another example. There is a "world" map that shows a number of key locations without a load of extra stuff, and a few smaller, more detailed maps for locations where most of the action takes place.

That kinda brings us back to the beginning. Attractive or not, a map that fulfills the its purpose is IMHO a good map.

Hope that helps a bit.