Major lit-geek squeeage ahead. First, the attention paid (especially by Dean) to language throughout the ep.
(1) the "old friend" conversation: "am I speaking a language you don't understand?"
(2) first conversation with Cassie about the way she broke up with him: "that's not exactly what you said"
(3) the insurance men conversation: "are you even speaking English?"
(4) Dean going alone to Cassie's: she says she's "trying to find the words" for the story
(5) "every time we started to get - what's the word - close?, you backed off"
(6) second discussion of the break-up with Cassie: "those aren't the words I used!"
(7) the fact that Dean was using an extended metaphor for the way he felt she treated him and that she called him on it
(8) "I couldn't lie to you"
(9) that Dean listens when Sam says he misses conversations that don't begin with "this killer truck" and amends it to "this guy Cyrus"
(10) that Dean listens when Cassie says she doesn't respond well to his authoritativeness and he reiterates his command, making it a request and ending it with "please?"
(11) Dean incredulously repeating Sam's "maybe"
(12) Cassie saying "Goodbye" while Dean opts for the less permanent "I'll be seeing you"
All of this (and more too! see below) made me believe that Dean once loved this girl. The way he couldn't get her words out of his head even after years apart says a lot to me. And it killed me because he really made the attempt to try to step outside himself and his bizarre life and put it into words for her, just like she needed. Only she couldn't hear him at the time. I did like that she apparently felt enough for him that years later she remembered what she couldn't let herself believe before.
So that's the spoken language portion of the ep. The body language was killing me too. The way Dean and Cassie, when they were finally alone together, were doing the 8th-grade-dance stance of leaning against the wall and making eyes at each other (I really liked that the camera pulled back so we saw how much space they left between themselves). The fact that Dean shoulders his way in while Cassie's still asking him inside. The fact that twice Cassie busies herself preparing a hot drink for Dean and that both time he accepts it and sets it aside (though Sam sips his like a good boy) like he knows she's avoiding eye contact but he's not about to do the same thing. And on top of that, his hesitance with her, the way we can see him coming to certain decisions. Just before he asks "why did you ask me where Sam was?" his scattered emotions are plain on his face. Plus the way he falls back on the bed with her on top and then rolls them over and then keeps her close when he sits back - not just hot but believable too, in that once he's made the decision to return the relationship to what it once was, he wants to keep them pressed together for as long as possible.
As for the plot, I thought it worked. Not because the truck itself is scary (though I was getting worried about Dean and the Metallicar) but because I thought the ep did a nicely understated job of revealing that the real horror of the situation was not the vehicular deaths but the complacency with which those deaths were met. I'm ashamed that I cannot remember the man's name, but I kept remembering the highly publicized but not at all isolated case of the black man who was tied to a truck and dragged for miles and bled to death. And Dean's "why didn't you call the cops?" line was so uncharacteristically stupid that I started wondering what its purpose was (we know Dean's not stupid). I wonder if Dean, cut off as he's been from normal society and seeing other non-human "races" all the time, has very little understanding of racism. If he and Cassie got any hostile looks or comments while they were together, I think it's entirely possible that he would have chalked it up as a reaction to him rather than them. After all, he's used to being the stranger who shows up whenever something weird has happened, so he must be used to unfriendly looks and words.