I think the biggest difference is "conformity". For example; I worked with a friend on his Harley before I ever worked on cars. I was intimately familiar with them by the time I was 15 or so. Everybody I grew up with that rides has a Harley, and the ones that don't are saving for one. Imagine their shock when I showed up to go out riding on my Goldwing.
http://www.robertbard67206.com/images/My_bike-1.jpg
Note: This is my second Goldwing. After taking my first one on an extended road trip, from Wichita, KS to Dallas, TX, I won't have anything else for an extended ride.
Anyway, the point is, I ride what I like, not what my bro's like, or what people on the forums tell me is cool. Hence the "conformity" thing. Customizing cars is supposed to be about "custom". If everybody's cars look the same, what's custom about that?