Ranting from an old man
1) It can get frustrating when trying to get a shot of a car without people getting in the picture. Not anyone's fault, but just when you are ready to take the shot, someone will walk right in front of you.
2) Car owners sitting right next to their cars in lawn chairs the entire day is a pet peeve. They seemingly have zero interest in anyone else's car other than theirs and get in the way of a good picture.
On the other side of the coin, talk to the owners and you possibly may get a good background story. I love hearing about cars that have been in the family for generations.
3) Too many hoods open. If you have a very exotic or unusual engine it is one thing, but most are not. Ruins a good picture of the lines of the car. Most owners will close the hood if you ask.
4) Too many props I personally find a little over the top. Models of the car sitting all over the engine, stuffed animals everywhere etc. I once saw a Corvette that must have had 30 skulls and skeletons plastered everywhere which bordered on ridiculous.
5) Concentrate on cars you like. I find the unusual models much more appealing than the common popular cars. I'll walk right on by 1957 Chevrolets to look at something a less common.
6) Be cognizant of the sun. You don't want your own shadow getting in the way. Pictures at high noon are also not the best either.
7) If shooting close up details like hood ornaments, you would prefer a shallow depth of field (large aperture) to get a good background with bokeh. For shooting an entire car, you want the opposite