I don't mind that the season was mostly filler. If it had been all classic episodes like season 1+2 it would have been awesome. I mind that the season was really bad filler.
The problem with the direction the show has taken can be boiled down to two problems.
1. Lack of subtlety
2. Humor becoming a priority
The first problem manifests itself in to major ways. The first is that the supernatural creatures have no mysteriousness about them. We don't learn about thier evil plans secondhand from the witnesses anymore, allowing the audience to connect to the human side of the evil and fill in the blanks with thier mind (something that always makes fear the most effective). Instead we see every aspect of the monster's lives, from over the top murders the commit, to thier mundane details of thier everyday lives. They aren't mysterious beings living in the dark corners of the world, but glorified mobsters that live in mansions and have jobs. This robs the creatures of all fear the might inspire. It also applies to the humans as well, as all of them are over the top archtypes rather than actual humans. No one feels like a real person anymore, but rather a sloppy cliche shoved in your face.
The second problem is definatly the humor. The simple reality is you cannot be both scared and laugh at the same time. The writers frequently admit they make many choices based on what they feel is funniest rather than what makes the story most effective. You get the sense that the writers spend most of thier energy trying to make eachother laugh rather than really tell a good story. Frequently dramatic or otherwise effective scenes are apsolutly ruined because of a cheap joke. This is at its worst when it applies to the monsters themselves, because it takes away from the fact that they are evil and deserve to be killed. The most unforgivable example was the James Marsters/ Charisma Carpenter episode where Dean and Sam actually let the two monsters (who had spent the entire episode killing innocent people) go free because the writers chose to write the climax around the gag of Sam helping them work out thier relationship.
