The line-up is a lot less musically diverse than people say it is. By my count, and obviously its not perfect coz I don't know everyone that's playing, there's a lot of indie, pop, rock, techno, d&b and house. There's always bit of psytrance, some breaks, a bit of breakcore if i'm lucky, various other microgenres of generally UK-centric dance. Obviously this is not an exhaustive list. But overall some absolute behemoths of musical genres are badly represented - US hip hop, dancehall, metal, jazz, punk. There are bits and pieces of these around, but not a lot, and without going into detail the stuff they do book in these genres often fits into a particular mold. Some more niche genres are also overlooked - ambient, experimental, noise, everything covered in The Wire. Looking this year I see almost no drill, as in previous years there was never any road rap, and no amapiano either, though for all of that I might just not know the artists on the bill who play it.
In general Glastonbury isn't that interested in pushing musical boundaries and tends to play it a bit safe. No-one ever agrees with me about this, but I still think I'm right. It's very rare that I'm wandering around glasto and hear something that feels new to me. I'd love it if it was different, but the reason they tend to be a bit conservative is probably that they know what their audience likes, which is fair enough. It works.