What security is lost with an "Open" SSID for a wireless network?

We have Aruba wireless and I have been working on configuring a SSID that will perform Mac Authentication for our staff devices. I know Mac Auth is not completely secure, we have ClearPass on the roadmap but this is just to tide us over.

What am I losing but not also adding a WPA2 password to the SSID? From searching around Aruba says that "all" traffic would be unencrypted, but I assume https etc.. would still work, so i'm just trying to get a better idea of what types of traffic would lose encryption without the password.

Thanks.