Steve Knightley is on the Croissant Neuf stage, as is the Boat Band.On the Avalon stage there's Edward II, Seth Lakeman, Eliza Carthy. Tinariwen are on the Pyramid Stage, also they are playing on the Jazz Stage with Tunng. Baaba Maal is on the Jazz stage too.
Acouistic Satge has Fairport, Penguin Cafe and Kilfenora Ceilidh Band(I suspect not with the line-up I have on my record, it was quite a while ago!)
To diverge slightly from the folk/roots category, my personal must see favourites will be Tom Jones and Spinal Tap on the Pyramid Stage.
As always, I have no doubt the most interesting music will be people I've never heard of, that I come across in some obscure venue that I pass when on my way to see someone else. Which is why I always miss the main acts. And the most fun will be with Biggles Wartime Band, round the fire till dawn and see who else turns up to jam. That happens at a Secret Location.