Yes, it's all about the weather!
For Glastonbury I'd plan the following, and then pack last minute depending on the weather forecast:
- summer outfits to float about in if it's hot (for me, that'd be a strappy summer dress with decent pockets).
- a sun hat, shades, tonnes of lotion
- comfy shoes x 2 if you have space
- outfits for cold / rainy days including wellies
Either way, lots of layers as it'll be cold at night and you'll probably want to sleep in a jumper.
Warm, waterproof coat.
- No jumpsuits or playsuits as you really don't want to have to deal with a jumpsuit in the Glastonbury toilets.
In terms of style, as the PP says, anything goes! If you enjoy getting dressed up, go for it! If you have a playful side, revel in it! Or if If you'd rather simply be comfy, then practical shorts & t-shirt or whatever is totally fine! Many people wear pretty ordinary clothes (they're just not the ones who get photographed for the articles on festival fashion)
Don't bring anything you'd be devastated if it got trashed / lost. Be aware you will need to carry your stuff a LONG way from the carpark to camping so try not to bring too much.
Be aware that Glastonbury is a place of extremes - there's not a lot of shade if it's sunny, and the drainage is poor so it gets muddy quickly if it rains (don't let that put you off though,!)
Google images of Glastonbury people, you'll find lots to look at.
Finally, not a fashion tip but to things for newcomers... get there as early as possible (Thursday if you can! Or Wednesday even - not kidding!) as the decent camping spaces get filled up fast! There's plenty going on before the music on the main stages starts. And it's great to be there and settled and feel everything build up to when when the music does start.
Also, while it's good to know when the bands you want to see are on, just pick one or two a day you absolutely want to see, and be a bit flexible about the rest. There's SO MUCH to see and do at Glastonbury, and the site is so big, it's absolutely not just about the bands on the main stages. It can be a bit stressful if you try to stick to a rigid plan to see specific things. Much more fun in my opinion, to go with the flow and enjoy what you discover along the way.
Have fun! I'm jealous!