I certainly don't have the massive amazing collections some of you guys have; fortunately (???) I only have a few cars, and showing them off to their best is what I look for in a display. I need them to be behind glass, and limit the dust that will get on them as much as possible. They also have to be well-lit, and the cars need to have the tires in the air to avoid damage and flat spots.
I got the idea to place mirror sections under the cars from full-size car shows...some show car enthusiasts have mirrors under their cars to show off the undersides of the chassis. If you put all that work into what's beneath the sheets, might as well showcase it. Ikea had 4-packs of the mirror squares for $10 a pack. I used dollar-store rectangular clear acrylic containers with felt tape on the tops to be the chassis stands.
The cabinets are nice units that I was able to convince my wife to share double-duty as displays for some cars and the fine kitchen stuff. At least the blonde wood unit is in the basement and I have the whole unit for my display needs. Eventually my OptiWTF (half-built) and the "Red Tide" Lucas Agitator will fill up that display when they're finished...and that'll probably be pretty much it. (Famous last words?)