|Poster by Craig Ward - Words are Pictures|
There appears to be no clear definition that separated good typography from bad typography. Generally analysing the quality of someone’s use of type is seen to be a largely subjective pursuit. Overall it will be the client to decide if the use of type is sufficient to communicate their intended message. The issue is that bad typography can still communicate a message and the client might not understand that there can be a better way.
The general consensus is that good typography will communicate the required information but will also invoke emotion suitable to the message. Good type will fit the message while bad type will stand out and create a feeling of confusion. To the untrained eye good design will be invisible.
If you are looking for some guidelines to follow for good typography you can find them here.
It seems to be that once you have the basics, its ok to break the rules sometimes.