Doesn't have a Zeppelin sound? Who said that? Back in the days when these guys were putting out Zeppelin albums do you think they asked if it had a "Zeppelin sound"? Many fans were disappointed when Zeppelin 3 came out because "it didn't have a Zeppelin sound". That's not what these guys are about.
That said, I too was disappointed a bit by Walking into Clarksdale. I was expecting something else from Jimmy. But after listening to it for awhile a few of the songs really grew on me. It doesn't have the ooomph for real commercial success but as one poster noted some of Plants lyrics and feel have a poignant touch to them.