It's not the first largely covers album he's done, Terr, there was - ignoring the Krauss one - this other one a few years back with Morning Dew and Hey Joe, I forgot the title. This new one reminds me of that.
For all his prowess as a singer, I never thought of Plant as much of a songwriter, he always needed co-writers to fully blossom.
And granted, Fate of Nations was a fine album, but that was almost 20 years ago. It was - for Plant at least - overtly commercial with songs such as 29 Palms, but I don't think he wants that type of music and that type of production anymore. His major influence in the last two decades has been American folk and the production of his solo work has become correpondingly more unrock. Plant today is closer (and wishes to be closer) to Lyle Lovett than to, say, even the Yardbirds, much less Led Zep.
I respect that, a man must do what a man must do. If he would have enjoyed himself being the Led Zep god again at the O2 concert or if he would have needed the money, he would have reunified Led Zep then and there. But he didn't, which means (a) he doesn't think that banshee wailing songs he co-wrote as a young man befits a man his age, (b) he has enough money, and (c) he doen't need the adoration of millions of Zep fans to feel good today.
It's ok to be Mick Jagger and the Stones are your family and you've decided to die with your boots on on stage. But it's just as ok to be Robert Plant and say "I was a huge part of what made Led Zep special, but that was thirty years ago and I am something different now and have no urge to go back".