This might be a controversial one, but... don't try to be a beer festival at the same time. I understand the beer can be lucrative for preserved railways but the type of idiots it attracts are undesirable to other attendees.
And my first two suggestions as soon as I saw the title of this thread were always going to be, and remain, "a decent real ale bar on the train and a decent pub on or near at least one of the major stations."
In addition to that, I would also express a preference for a frequent service and a decent length of end to end journey, as mentioned by others above.
I'd also add a varied lineup of heritage locomotive types; it's only personal preference, but I feel quite strongly that modern loco classes attending galas rather detract from the preserved railway experience, even if locos in seventies and eighties guises hauling blood and custard liveried mark 1s through stations restored to 1950s condition can hardly be considered authentic in itself!
And while I appreciate that there are valid reasons to charge a premium during gala events, particularly if there are large numbers of visiting 'guest' locomotives to pay for, I do appreciate it when the fares aren't extortionate. The Great Central diesel gala next month, for example, looks like very good value for money.
