What difference do you see between "a key to success" and "the key to success"? Probably, the difference might be whether the speaker believes there is only one way or multiple ways to success. But there are certainly at least more than one key to success. Then, why people tend to speak of "the key to success"? Are they simply emphasizing their point or are they being overbearing about their point? Or are they just pointing to the fact the key they present is single?