What's the main difference between Pohlig-Hellman and RSA?
The main difference is that Pohlig-Hellman is a symmetric cypher, while RSA is a public key system.  This is because, with Pohlig-Hellman, anyone who knows the encryption key $e$ can compute the inverse operation (because the 'decryption' key $e^{-1} \bmod p-1$ is easy to compute), while the RSA, someone who knows the encryption key $e$ (but not the factorization of the RSA modulus) cannot compute the decryption operation.