I agree with all of the above..
But specifically about the music, I'd want to have a sound system that had speakers everywhere rather than 2 in one spot blasting out trying to reach everyone.
With a bunch of speakers, you could turn the frickin volume down and then EVERYONE could have an enjoyable time - not just those out of range of the ear busters.
Now I like loud music but not when I'm trying to talk to someone about a car.
Something else I'd be sure to do is tie the show to a charity or a few. Even if it's just 50/50 draws. The charity aspect makes it easier to get some prizes donated and the prizes draw more people and cars. I think it will also keep organizers and volunteers more committed to keep things rolling.
If we're talking a big car show, then there has to be some special interest cars and that usually takes some money. That'd mean registration fees and spectator entry fees. If done right, that can work well.
MOPARFest does all of that and is tied to a few charities and it works very well. It brings in a lot of sponsors and vendor donations. Heck, they give away a car and a crate engine - your name goes into the hat just for having a car on the field. There's a ton more prizes for participants and spectators, too.
A good sized swap meet area is mandatory.
I'm on the fence as to whether the knick-knack and kitsch vendors should be in their own area or mixed in with the parts vendors. On one hand, it makes getting through the real parts a lot easier. On the other hand, if you're there with spouse and kids, it's good to have it mixed so you can all walk through together and there's something for everyone to peruse....
And do the car show at a huge hotel with a room included in the registration fee so I don't have to camp or surf a couch...