I'd take somewhere with real bathrooms over porta-potties any day. That would be my #1 deal-breaker. Anytime you go to a big outdoor venue for a weekend-long event you get to stand in mud, have a panoramic view (translated: far away from the stage), and pay too much for beer. That's gonna happen anywhere. Having to wee in the dark, stinky pee-closet is enough to keep me home.
I do have to give major kudos to the Voodoo Music Fest in New Orleans; I went over the weekend and even though I had to deal with the nasty porta-potties, the venue (City Park) was awesome, 4 stages perfectly laid out, good flow of foot traffic, and most pleasantly, reasonably priced food and drink.