Difference between open authentication and WPA

If it's OPEN there is no authentication.

Ummm, sorta.

WPA includes 802.1x authentication as manditory. 802.1x is a replacement for the previous open authentication and shared key authentication.

For WEP, not WPA, encryption, there is either open authentication or shared-key authentication. Open authentication is a nice way of saying no authentication. Shared key authentication uses the WEP key to create a hash code used to authenticate the user. Unfortunately, shared key authentication has enough information in the hashed packets to recover the original WEP key and is therefore considered to be a security problem. In this case, open authentication is considered more secure.

However, using WPA eliminates the authentication issue as the built in

Fun reading on shared authentication security: WEPWedgie