Since the Apocalypse, which was more like a series of events than anything else, Supernatural on a grand scale hasn't been that all that exciting. Angels and demons pop up to do battle on occasion, but it always felt like isolated instances rather than a grand battle for Earth. Blame it on the lack of budget for sweeping CGI shots or whatever; it didn't seem like too big of a deal.
When Castiel sucked up the souls, however, it was a big deal because it dealt with a character we've come to love. There's significance to him as a main character, not just a random angel. Proclaiming himself God, Cas pretty much went off the deep end, leaving Sam, Dean, and Bobby behind and running off to do things that God is supposed to do. It turns out that Cas doesn't really have the right mindset, as it kills people left and right. The season begins with this shocking behavior right off the bat and it's all very scary.
After some plot development in the middle, with the return of Death who is always fun, Cas gives up the souls, surprisingly. I had expected Cas to retain his new powers for a couple episodes, but he loses them quite early, allowing us to see Cas's regret over what he'd done. If that had been it, I'd be a little disappointed, but there is another twist afterwards, the Leviathan staying in Cas's body and taking over. Needless to say, Mischa Collins is awesome in the final scene.
There is less focus on the Winchesters since Cas is doing his thing, but with Sam's wall broken, he's having problems. Lucifer shows up and tells him he's still in Hell, which probably isn't true, but must be psychologically harmful to Sam.