Well, that settles it! We need a sentient/human car episode!
Here's why the Impala is awesome: Sam and Dean are not the first two paranormally pestered siblings to cruise in a '67 Impala.

Years ago, in 6th grade (circa 1988), I wrote a novella based around a brother and sister pair. Two years later I re-wrote it (typing it out on a Commodore 64), and this time I gave the older sibling (the brother) a car. I based the novel in 1979, and since I wasn't born until 1977 I asked my dad what would be a good car - reliable, not too flashy, but lots of cool factor. Can you guess what he said?

The only difference was that, in my book, I envisioned the car as Marina blue instead of Tuxedo black (though I don't recall if I specifically mentioned the color).
Synopsis for the story? The brother was cursed and the sister was trying to cure him, there were demons and angels battling it out for his soul, and Hard Rock/Heavy Metal music factored in (including a poignant scene with appropriate soundtrack). There was also a mysterious, aged tome detailing the nature of the curse. I shitteth thou not! I can't tell you how many times I've had to lift my jaw from the floor watching SPN!
So, you want to know what makes "Baby" (I call it "Black Sunshine"

different from other cars? The fact that '67 Impalas have been the chosen ride of supernatural siblings battling demonic evil [and listening to great music] for a couple decades! \m/ >.< \m/
You know, my hard copy of that book vanished! I've always raised an eyebrow at this show just a bit. o.6