Any episode that threatens to introduce a long-lost third Brother Winchester is treading on thin ice from the get-go. I distinctly recall the hue and cry when the first inklings of this episode were strewn across the internet by distressed fangirls. They shouldn't have been worried. How many times have the earliest spoilers turned out to be woefully devoid of context?
I knew that all would be well once Sam and Deam met with Adam at Cousin Oliver's, passing by the Fonzarelli Water Skiing poster on the way to the table. The episode was filled with references to past "jump the shark" moments, and the writers used the expectations of the audience to pull off some wonderful twists.
The interesting thing is that the existence of Adam shouldn't come as a surprise, if one thinks about it logically. The life of a hunter lends itself to sowing wild oats, so to speak, and if Dean is a reflection of John, then where else would Dean learn the ropes? We've already seen one episode where Dean thought he had fathered a son on the road, so the odds were favorable for more Winchesters out there.