I always liked Hats Off as a weird, off-beat, unexpected ending to such an eclectic album. It is raw and has a mysterious "old" sound. It is so different because it is just Plant and Page, and both of them are tearing it up. So anti-commercial. To me it is the perfect send off to this record.
It is so Zeppelin that they took an excellent, very commercial song like Hey Hey, left it off the album, and put it out only as b-side.