I loved how the plot played out, from the trail of clues, to the reveal it was Amazons, to the twists the detective was one of them and it was the daughters who killed their fathers. The intrigue was extremely well paced and put together.
I really like that this show doesn't give us simple black-and-white, it's all messy shades of grey. The boys don't always get the easy choice between right and wrong, they have to make tough choices between wrong and more wrong. And I loved the mirror image here, where first it was Dean who had to do the deed because Sam was too close to Amy, and now it is Sam who has to kill Emma because Dean can't gank his own daughter.
Dean's evolution with women is interesting: post-Lisa we haven't seen him have any one-night stands. He tried in Defending Your Life, and was a ball of nerves trying to talk himself into it while waiting on the sidewalk for her before getting kidnapped. In this episode, Lydia is the one in control, and her predatory and assertive style is great foreshadowing that she is actually an Amazon. Then the next morning we are treated to a reversal of roles as erstwhile love-em-and-leave-em Dean has taken down her phone number and calls her only to be given the brush off and be called "Don" instead of "Dean." I really appreciate the fact that the characters evolve over time in reaction to their life experience, and clearly Dean's year with Lisa has had an impact on him.
I also like the contrast between how Dean and Sam are coping: Dean destructively through drinking, carousing and repressing, and Sam constructively through work, exercise and expressing. Two details that emphasized this was the booze flask Dean was never without, and the fact Sam was always the one driving. Even though it's not his Baby, Dean doesn't give up the steering wheel easily. That Sam is the one driving speaks to Sam being more in control of the two.
Their conversation in the car at the end was powerful, marrying funny banter and heartbreaking lines. "Just don't get killed."