Roads to Safe Places

(I hope you don't mind random people indruding in your entries, but I found this a very interesting question.)

I think Supernatural shifted a lot in his targeted audience. The first three seasons were marketed with the catch phrase "Scary just got sexy" and had those two guys be randomly seducing the girl-of-the-week and do manly things like killing and driving and fighting, and pretty much made most female characters so bland that they were easy to be fill-in characters for self-projection (for example Jo, who gets her damsel in distress moments and banter with Dean, or Bela). As far as I remember the only slash fanbase back then shipped the brothers. That changed with season four and Castiel, of course, and also that was the time when the show became much more self-aware and ironic, in a way. Even if they might not have intended to specifically please the slash fanbase with the introduciton of Castiel, they surely noticed it and upped it to eleven.

In that episode where they are visiting the Supernatural-Convention, they have the panel "Homoerotic subtext in the novels" (between the brothers, as Castiel isn't a character in the books at that point), you get a good idea of just how self-aware they are. While the characters react with "That's just sick.", the actors and writers love to bait the fanbase with it, especially now with Castiel (things like "We are going to pay their relationship lip service" and other gems). I mean, the series has an in-series slasher fangirl who writes Dean/Sam fanfiction.

I guess you can say that in the beginning, the show was just aimed at straight female watchers who were intrigued by the whole supernatural aspect but wished for something scarier and grittier than the Twilight franchise, but over time it developped into a show specifically catering to the fanbase - mainly slasher fans, but also vague enough to please a more mainstream audience. You have both boys get a bit of family life and relationships with females, you have the tons of subtext (also with that whole Luzifer and Michael thing), you have totally insane meta episodes where no fourth wall exists whatsoever, and then again you have episodes filled with drama who want to be all srs bsns.
In a way this show is a good example what happens when multiple showrunners extend a five season show into nine seasons and always have an eye on the way the fanbase reacts in order to shift the direction of the show to their pleasure.

The showrunner of season seven was a female, and she was hated a lot in the fanbase, since she killed Castiel off, only to bring him back having a wife (and amnesia). The current eighth season caters a lot to the Destiel fans again, and, well, they enjoy it.