A good translation is one which:
is delivered on time and produced by a professional translator who is a native speaker of the target language, has an excellent grasp of the source language and a thorough understanding of the subject matter;
does not contain any linguistic errors: grammatical and spelling mistakes are unthinkable;
is complete: words and sections of sentences are not omitted, unless they are superfluous;
is produced according to the original layout (with the exception of PDF or paper source texts) and uses the appropriate terminology;
contains the client’s preferred terminology (where it is available to us);
is drafted in the preferred linguistic variant (e.g. US vs. UK English, European vs. South American Spanish or Portuguese);
conveys the message from the source language in the target language without loss of meaning;
does not contain any interpretation errors;
is coherent in terms of terminology, style and register;
does not 'feel' like a translation.