I've often wondered myself - I suppose in my case it's a bit of everything, i.e. all the factors you mention (who's on, date, location, probably in that order).

It depends on whether I'm playing or just a member of the audience, of course. I assume you are interested in the latter. I try to go to ones featuring guests I like to see perform, and hope to make new discoveries too. The main problem is when festivals clash date-wise, sometimes making the choice difficult.

Another factor we consider is the quality of the facilities available - camping, availability of pubs or bars serving decent ale, etc. One or two I've been to have been spoilt for us by the shortage of toilet/washroom facilities in the camping areas, resulting in huge queues at peak times (and of course in the festival site itself); or the lack of decent ale at the bar, rip-off food prices, etc.