The main feature of a quantum key distribution (QKD) system is its ability to provide secure communication channels through the principles of quantum mechanics. It leverages the properties of quantum bits (qubits) to distribute encryption keys securely between parties. QKD ensures that any eavesdropping attempts would disturb the qubits, thus alerting the users to potential security breaches. This method enhances the robustness of cryptographic systems, making it nearly impossible for unauthorized entities to gain access to the communication without being detected.