Never stage managed a music festival, but I have worked on them in other capacities. You really need to find out if you are stage managing in the sense of a theatre stage manager (ie pulling all technical elements together and organising everyone) or more of a manager of the stage crews. Anyway....
Organisation is key.
Unlike theatre, sound plays a much larger part in music festivals (duh...) than lights, however as a general rule, both are much larger than your standard theatre rigs (West End/Broadway excepted).
Unless your festival is controlled by sound nazis (very unlikely) every band will have different pieces of equipment used for their set. And it is not just "different guitars", but often different drums, different amplifiers, different microphones, different effects pedals, different monitor arrangement... everything.
Basically, you need to know what each band wants for their set, you need to talk to your sound techs (you will probably have a monitor engineer and a FOH engineer) about which channels things will be on, and you need to work out how you are going to get everything off stage, and the new set on stage within fairly strict time frames - no more than a few minutes usually.
Then you have people management - if you have 10 bands, with an average band size of 5 people, that is 50 people. If this is an outside event, that will mean they will probably be located off site, so you need runners, you need to guess how long it will take to get the band backstage and how long it will take them to get ready once there.
Communication is another big thing. There are scores of people over a large area - you need to know that they either know their jobs well enough that they can make their own decisions, or that you can contact them to tell them what to do in special situations.
The major thing to remember, I suppose, is that you do not have the rehearsal time that theatre usually provides. Chances are your mix engineers and you LX engineer will be used to this sort of situation, which helps, however you need to be keeping everything moving - sound checks, lighting checks... Remember, these are not times for the bands to practice their sets, they are to provide the engineers with a sample to allow them to present the bands in a good light. Get them on, and when the engineers tell you the mixes are right, get them off. Bang, Bang, Bang.