I was thinking about this quite a bit this weekend. Consider that a good headliner needs to do all the following:
1) Be enough of a "name" to sell tickets.
2) Not be too expensive to book.
3) Be accessible to the festival-goers who don't really know your stuff
4) Make it a show- put some effort in (how much time and effort had PJH and her band put into that set on Sunday? Brilliant!)
5) Have enough strong material to keep it going for an hour and a half- in most cases this probably means having been around for long enough to release 3 or 4 good albums. This is a real stumbling block for a lot of potential headliners.
6) Be in line with the sort of thing people can expect from a festival (probably best not have a metal band at the top of the poster)
For Green Man, this limits you to maybe 15-20 acts, of which most won't be available any given year. Tough, isn't it?
I did see Future Islands and while they weren't *that* bad as a band they clearly didn't work as a headline act. PJ Harvey very comfortably ticked every box. Past headliners who have fitted the bill best have been the Flaming Lips and Super Furry Animals.
I only saw a bit of the Shins' set but from what I saw they'd have been an inspired choice for Friday headliners. British Sea Power could also have worked, I reckon.
I'm wondering now that the festival is easily selling out each year (as opposed to selling out a week or so in advance) whether we could expect point 1) to be less important and we could have the potential for some really inspired more leftfield choices. Not sure who, but if you're selling the tickets anyway you don't need to spend huge amounts on a big name for the sake of the name.