An interview with Supernatural's Eric Kripke on why he's stepping aside as "day to day showrunner" and taking an advisory role on the show:
ERIC KRIPKE: We were reaching the end of this five-year story line [so] I thought the timing was right. I knew that we were closing this chapter and opening a new one. It felt like it was the right time to take a step back and focus on new projects, but still keep my grubby little mitts in the show. It was a lot about Sera and her enthusiasm and her ambition. I really think after five years of all of my crap, to have someone who has a fresh perspective and a fresh energy on these characters and this universe is healthy for the show. Supernatural has always been a show about reinvention. We try really hard not to do the same thing. I thought that Sera’s [increased] involvement really helped guarantee that this season is going to feel a little different, a little fresher. She has a different sensibility.
Rest of interview here: http://ausiellofiles.ew.com/2010/07/15/supernatural-season-6-spoilers/
Beware of Season 6 Supernatural Spoilers