There are many options available for a pimped out ride. You can choose how much turbo boost you have, how many ejection seats, different kinds of lights, trims, sound systems, drink dispensers, and windows.
Perhaps the coolest thing is that it allows you to purchase an onboard companion. At first I was going to go with a "Hot Night Elf Girlfriend" who occasionally cast Blessing of Might on me, but I've since decided to go with the "Onboard goblin engineer for emergency repairs." Mainly because I'm a bad driver and have a tendency to smash into other people.
I'm a little disappointed about the cost of pimpage however. At around 30,000g I think it'll be cost prohibitive to nearly everyone. Only the pimps of pimps will be able to afford it. It's really a slap in the face.
The cool thing is that once you pimp your mount, you also earn the title "pimp," which can be strangely linked in chat. When I logged onto the game this morning there were several guild leaders of top end raiding guilds spamming trade chat, placing the world "pimp" after a certain word. If you don't know what I'm talking about, just login to the game today and watch the rich folks laugh with glee as they spam trade chat.
Overall a cool new addition to the game! Our sister site Massively has also covered this addition.