Here's a few thoughts, please keep in mind they are just my opinions.
-A good field: you can't have a decent game without a good field. The Edmonton field that I played a couple 24 hour games at is amazingly large and very well laid out. The Tamran field in Charlotte, North Carolina is equally huge, but also very hilly and it makes for amazing over-night games too (only played two there)
-A good scenario: Shooting & re-spawning is great, but it gets incredibly borring when you are doing it for 24 hours with no specific objectives. Get a good story-line, plot, even cast and voila, you have the potential makings of a game people will remember (and want to come back for)
-Good pricing to attend: The NC games were $40USD which at the time ('99 and '00) was INCREDIBLY expensive, while to attend the Edmonton 24-hour games was something to the tune of $25-30 at the time - very reasonable. People will lose interest if you over-charge them.
-Gimmicks: Having gimmicks is always a frill. Look at the big games in Mantiba, they went so far as to rent a helicopter to fly in troops. Blasyman (a user on this forum) has a custom made mortar, .50 Browning Machine Gun and other 'toys'. Chalcus (also from this forum) is a prop designer and has made some amazing props for his local games.
Just some munchies for thought.