One of the reasons cymbals generally sound better in the car is because they're mainly composed of high frequencies and tweeters are usually placed in the car at or close to ear level. Tweeters reproduce mainly high frequencies.I notice the same jump when I play a CD in the car. It's only little things like cymbals sounding better, but it's enough of a difference to know that I'd never want to have mp3's as the only option.
High frequencies, unlike low frequencies, are more directional, so if you stand right in front of a tweeter your ear will capture a lot of high frequencies. If you step a little bit to the side you will notice how less high frequencies you'll hear.
That's why studio monitor speakers are generally positioned in a way that the tweeters are in line with the ears.