I think that series deserves once more a bit of exposure.
Well, finished watching season 7. And I must say I quite liked it:
From season 4 on, the whole trend was getting imhO too big, too drama-y.
The coming apocalypse was just a too dominant theme. Also, there was hardly anything that could top the battle between Lucifer and Michael, and the writers do not really want God to take stage (which I can understand: "Is there a supreme being" is the last big question....should it be answered, the whole series will be deprived of much mystery).
Now, season 7 established the series early, more light-heated approach. A nice "let's not take things too serious" attitude. E.g. in the Amazon episode: It was quite a surprise that the villains -though being above human- suddenly could be killed by a single bullet. Also, the ending even had some childish, juvenile undertone "Dean, you killed my friend, so I killed your "daughter"....nanana...". Black in humor, but with those deliberate childish undertones.
The Leviathans turned out to be excellent villains, and it was their earning that the series got "back on track".
- They featured a non-human appearance (let's face it: The demons and angels became too human in the end).
- Despite being Über-Monsters, they were not handled as such. They were like the early demons: Though, strong, difficult to kill...but not undefeatable, and with enough weaknesses to give them the business in several ways (keeping the conflicts interesting).
- Also, the masterplan was not as over-epic like the Apocalypse. The Leviathan's plan reminded me a bit of a Bond Villain's approach: Sure they do seek world dominance, but it had some "tongue-in-cheek" undertones.
- And there were enough interesting "side quest" episodes. The Apocalyse was a too dominant theme. You basically nearly fast forwarded to see what happens. While the Leviathans could have been left standing a tad on the sidelines, to let the Winchester fool around a bit with other themes.
All in all: Season 7 removed some weaknesses that crept into the series since season 4.