This is the first time we see what hell hounds actually look like.
When Kevin tells them about the trials to seal the gates, Sam asks if they are trials "like Law and Order" to which Kevin replies "More like Hercules." This is in reference to the 12 labours of Hercules, meant to be "feats so difficult they seem impossible." The 3 trials are very similar to this.
When Kevin tells Sam and Dean that they need to kill a hell hound and bathe in its blood to close the gates of hell, Dean replies, "If this means icing all demons, I got no problem gutting some devil dog and letting Calgon take me away." He is quoting the 1970's slogan of the Calgon bubble bath products.
Sam finds a possible cross-roads deal and Dean says, "Let's go visit the Beverly Hillbillies." He is referring to the 1960's TV sitcom, The Beverly Hillbillies (1962), about the Ozark Mountain 'Clampetts,' who strike it oil rich and head to Beverly Hills with its "swimming pools and movie stars."
When Ellie sees Dean wearing glasses, she says that she likes the Clark Kent look. Jensen Ackles, who plays Dean, played Jason Teague throughout the fourth season of Smallville (2001), a TV series about Clark Kent before he became Superman. Furthermore, Jensen was considered for the role of Clark Kent, which eventually went to Tom Welling instead.