Write a comprehensive technical explanation of Quantum Key Distribution (QKD), specifically focusing on the BB84 protocol. Your explanation should cover: 1) The fundamental quantum mechanical principles that enable QKD, such as the No-Cloning Theorem and observer effect. 2) A step-by-step walkthrough of the BB84 protocol including basis reconciliation and sifting. 3) Analysis of potential eavesdropping strategies (e.g., Intercept-Resend) and how they are detected. 4) Current limitations and challenges in the practical implementation of QKD over long distances.