As Sam is drawing the tree from his dream at the beginning of the episode, Dean says "A fishing trawler found off the coast of Cali, it's crew vanished". This is a possible referral to Benny's (season 8) nest working off the coast of California.
There are no deaths and only one major injury in this episode. It makes it the first episode in the series with no fatalities. (Although the plumber is shown losing his arm in the garbage disposal, there is no police presence, none of the characters react to it as though he'd died, and he's well enough to try to file a lawsuit against Jenny, as we hear on the phone.)
When Dean mentions a Sacramento man committing suicide with three shots to the head. This could be a reference to Gary Webb who reportedly committed suicide in Sacramento with two gunshot wounds to the head December 10, 2004.
There are at least three Stephen King story references in this episode. The first was in the beginning when Sam first tells Dean about his visions, and Dean calls it his "shining" (The Shining). The second was the clapping monkey toy when the plumber is fixing the garbage disposal (The Monkey). The third was when Sam is trapped in the house at the end and Dean uses an ax to chop his way through the door, followed by a shot of him looking through the hole in the door (The Shining).
When Sam expresses unease about the house being poltergeist-free, Dean says, "Why? Missouri did her whole Zelda Rubenstein thing. The house should be clean. This should be over." Zelda Rubenstein played the diminutive medium in the 1982 supernatural horror film "Poltergeist."
This is the first time we see Dean express vulnerability and strong emotion: at the prospect of returning to Lawrence, being in his old house, feeling lost as to what to do, trying to reach his father, and finally confronting his mother's spirit.
At the end of the episode, Loretta Devine's character Missouri tells the boys "Don't you boys be strangers". This is in fact what they did as Devine has not made a re-appearance in the series since. However, it was originally planned for Missouri to be the boys ongoing advisor/parental stand-in, but Devine took on the regular role of Adele Webber on Grey's Anatomy. This opened the door for Bobby Singer to be added to the family.