I think it is fair to say that four things have to suit the person going though; music, venues, memories and value for money.
Personally, I prefer smaller festivals - that doesn't mean I don't like the big ones, with massive names headlining - I just prefer the smaller ones. Saying that, I do like the variety in genres you tend to experience at a larger festival, like Isle Of Wight or Reading & Leeds, you can go from one stage and listen to a folk band and then head to a different stage and see a pop punk band.
Venue will always be an added thing to decide upon, if it's easier to get to, around and stay at for anything up to a week. Often, they are easy, otherwise they wouldn't be successful and a lot of the big names in the festival list become a village of music and art lovers for a few days.
Every festival has to be able to offer you memories, whether it's seeing a new band, a favourite band, getting drunk and having a laugh with friends, you get the idea...
Along with all of the above, value for money is a really big thing, paying too much for not a lot isn't going to go down well, but you still want to be paying to see big artists and the overall feel of a festival. These new free festivals, are great locally, however, I'm not sure they're putting those little touches into organisation fully for people going to enjoy it... (I think realistically that's what I'm getting at with this post - I'd much rather pay a fiver to have a good day and enjoy all the brilliant music, when they should be on stage with friendly people running the overall event, rather than have a crap time for free).
I think apart from those big things, there seems to be a growing list of smaller things that seem to make up the overall picture of a good festival...
Weather, food, security, mobile phone charging, port-a-loos, tents, people partying at 3am in the morning... These things unfortunately can put people off festivals as well as add to the bigger and better things, but I want to know, what do you think makes a good festival?