There are many points mentioned that draw me to certain car shows, but I have a question first? Are there other attractions nearby? Shopping, antiques, things to do - then you can use that as a draw to the show also. If they are close, talk to the nearby businesses about being open during your event and offering something "special" for the car show participants/spectators.

1) Time: Not too long - 5 hours or less
2) Prefer grass but have done several parking lot shows.
3) Prefer a Top 20 show to ones that have individual classes. Also on a top 20 show, I like to see some trophies that are stand out trophies like: Spectator choice, Best Paint, Best Engine, Best Interior, Best Wheels - All of these are judged by the trophy sponsors.
4) Prefer shows were the Top 20 are voted on by the participants.
5) Like to see "Goody Bags" that contain dash plaques, coupons, Sample products, etc. In this way "everyone" gets something for coming to the show.
6) To get my wife to attend with me, there needs to be something for her. Craft items, Nearby shops, antique stores, etc.
7) While I like shows that have a specified year, just starting out you might make it an "Open" car show. Someone may bring their showroom new car to the show, but it will be another $10 in the band fund. Some of these cars get people looking at them too as it's like going to a car dealer to see the car and talk to an actual owner as to how they like it. Sometimes, the morning of the show is when I decide exactly which car I'm bringing. My 34 Ford Street Rod, my restored 1929 Model A coupe, or my 2003 50th Anniversary Corvette. The corvette can't come to some car shows, but is welcome at others.
8) nearby food vendors and restaurants. I can't do the greasy fried foods, so I like shows that have a variety of food things.
9) I like the idea of having percussion show or other things of particular interest. Have a hola hoop contest for the band and other spectators, other special events.
10) car shows with a sound system and music with periodic announcements make the day go faster too.

Sorry for my ramblings... but... Since you asked... These are what gets me to one car show versus another.