But what makes these cars make it to a poster on a 12 year old's bedroom wall? Is it that they are fast? Or is it that they are beautiful?

Ferraris are fast, Lambos are fast, but being fast doesn't seem to be enough. An Apollo Gumpert would run rings around any Italian horse or bull, but in my opinion it's just "plastic junk", and I don't think any boy would like this car on his bedroom wall. Besides, Alfa Romeo's aren't too fast, but still are the "poster kind".
So maybe it's not just speed., and maybe looks are the key. Look at a Miura, or any Alfo romeo, they look stunning, and make perfect centrefolds. But why don't any Audis and the BMWs fare in the fantasy list, many of them look good. And, what about a Pagani Zonda, it's definetely ugly, but I love it. And who doesn't like a Zonda? The ugliness, the wildness, the crazyness are all a part of it's charisma. It's ugly but still lovable.

So what is it if not just the speed or the looks that make these cars so special. I recall an episode of TopGear where Jeremy Clarkson said about a german car that it may be fast but lacks the soul. Maybe that's what makes a fast car a "fantasy car". It's the soul. It's the wildness of the Lambo's and the Zonda's, it's the perfection of the Ferrari's. It's not how fast the car goes, it's what goes into it besides technology.
In the words of an Indian Comedian, "..yahi to kalaa hai, baaki sab to vigyaan hai.." (this is what's art, rest is science).