I loved every single thing about this episode. The lighting. The music. The clothes (OMG, the clothes). I LOVED the slang getting in the way of each of them being able to understand the other. Other things I loved:
Sam teasing Dean, first about his "cartoons" and then about "being into Dick." Heh heh.
Dean's moved on from always picking scissors, and he still loses!
Dean knowing a computer trick Sam doesn't and being ambivalent about whether to brag about it or not.
Dean in rolled-up shirtsleeves, Dean in that suit, Dean in the shoulder holster, etc. Sam in the shirt with the dark red and green stripes.
Jody and Sam working together (and that's not a 'ship that ever crossed my mind before, but HELLO).
Dean's fanboy wonder at meeting Eliot Ness and working with him, to the point where he puts it in his note to Sam. Dean thinking he's not going to get back to Sam but using his movie knowledge to send him a message. Dean's fanboy nature being unappreciated yet again, no matter how it lights up his face. Sam's face lighting up when he found the note from Dean (ETA: And Dean addressing the note to "Sammy" *_*).
Nicholas Lea as a federal agent with a side interest in the supernatural.
For all that Dean refuses to share his feelings, he sure likes to open up when there's a father figure/mentor type around. Ness also made a great contrast to Frank in terms of why he hunts and what he gets out of it. "Who died?" "No one died, you morbid son of a bitch." Hee!
Kronos's warning about the future: since the boys changed the past (Dean being strangled) and the present (Sam's name carved into the wall), they probably don't have to worry too much about changing the future.
Okay, so the actual plot wasn't all that stellar and had a lot of neatly-timed coincidences, but there was so much squee that I don't care for once. Loved it!