This thread is fine where it is, since the lineups are the topic of the other one.
I'm a 4 year vet, and what you bring depends on who you're going with and what you plan on doing.
For starters, water, canned goods/dry food, sunscreen, and a few changes of clothes. Be sure your camping setup is focused on shade. You honestly won't sleep as much as you think due to the heat, and there are very few trees to seek shade under. You can get battery powered travel fans from walmart, and they lasted almost the entire weekend for me. Kept me cool enough to rest at night.
Anyway, the idea is to bring enough food, water, and amenities to avoid paying 3-4 times as much for that stuff at Roo. Yes, a burger and fries will cost AT LEAST 10 bucks there. Protein bars and jerky all the way.
Moneywise, you should bring anywhere from 200-400. Honestly, this is purely dependent on how many party favors you want to indulge in as well as merchandise. OH, speaking of merchandise... wait until the last two days to buy anything. I've vended festivals. We tend to slash the price in half on the last day (who's gonna want a Bonnaroo 2013 shirt in 2014?).
I stress, though, water is super super important. You DO NOT want to drink the well water they have there... especially by day 3. It's brown. Seriously. Avoid that mess.
Just expect every day to be 90 degrees (index hovering around 100).
Most importantly, have fun and make friends. That's what festies are all about.