I learned today that some properties of the SPSite, SPWeb, and SPUser classes that are read/write in farm solutions are read-only in sandboxed solutions. I believe this is a complete list:

SPSite.Owner
SPSite.PortalName
SPSite.PortalUrl
SPSite.SecondaryContact

SPUser.Email

SPWeb.AnonymousPermMask64
SPWeb.Author
SPWeb.Created
SPWeb.LastItemModifiedDate