The creators of Supernatural only intended for the show to go for five seasons and culminated in the battle with Lucifer. I think we can all agree that the fifth season was the best and the final battle was memorable as we saw Chuck blink out.
Networks thrive on ratings and ad revenue, so when you have a show as good as Supernatural, you just can't let it go. Instead of ending it after literally beating the most devastating evidoer of all time, they continued on trying to piece their lives together after the near Apocalypse. I will admit that I thought season six was good, but it lacked the passion of the other seasons. It was off the plan, so everything seemed disjointed and hurried.
With the seventh season, it all seems to have fallen apart. There is not real forward momentum and the characters of the Leviathans are not developed. I don't hate or feel sympathy for these creatures, I simply don't care.
The interaction between Sam and Dean has become stale and well...whiny...boo hoo hoo. It's the same old problems they've been rehashing for three seasons. Now, they're killing off characters left and right. Rufus, Cas, and now Bobby. I hope it gets better before it loses out to the shark.