I agree that overtraining has been blown out of proportion.
Each person will have different limits. On todays posts there is mention of Josh McDougall running 18 mi per day in singles. To many people, that would be overtraining. It obviously isn't for him.
But we train to race, so you must self assess. If you are a 4:00 1500m runner, and you increase your training significantly but don't improve,and are constantly tired, then you may have overtrained. But the answer is to try something else, like faster intervals and more rest days, not simply less training.
However, if you read between the lines (and sometimes it is actually posted in plain English) most of the great runners that we aspire to be like (Hall, Geb, Webb, Ritz, Paula, Deena-the usual suspects) all train very hard-much harder than most of us have ever managed.
Therefore, my suggestion to serious runners is at least in one year or one season, err on the side of overtraining and see what happens.