...Monteith's death

http://www.billboard.com/articles/columns/pop-shop/6554034/kevin-mchale-glee-honest-interview-video

McHale (https://celebrity.yahoo.com/news/kevin-mchale-glee-jumped-shark-cory-monteith-died-232000255-us-weekly.html) says "it was just weird for us being there never mind actually having to be filming." He adds: "When Cory Monteith died. I don't think the show ever came back from that. It wasn't for a lack of trying, but the story was going in one direction with his character and he was one of two main characters of the show."

Cory Monteith's deteriorating health had a pretty negative impact on Season 4 as well. That entire season was structured around Finn feeling lost after high school, finding solace in coaching the Glee Club, and eventually discovering that he was suited to teaching. The Glee Club's major storyline that season was Finn molding the group of mostly inexperienced newbies into a team. If you go back and read some reviews, you'll see that the first half of Season 4 was actually considered a return to form.

But Monteith had apparently started using again during Season 4's filming, so Lea Michele and Ryan Murphy checked him into rehab about 3/4 of the way through the season. That meant that Finn was hastily written out of the rest of Season 4 with no explanation, which was awkward considering he'd been the driving force of so many of the season's storylines. The show obviously didn't want to close out any of those storylines until he returned, which resulted in Season 4's homestretch and finale essentially turning into filler with no momentum or resolution.

But then Monteith unfortunately passed away between seasons and that was that. Finn's final episode had had him start attending a local college for a teaching degree (while still co-coaching the Glee Club), so the show was obviously anchoring a lot of its future around Finn, particularly the McKinley High side. There are actually some early Season 5 promotional photos that prominently feature Cory Monteith (http://i.imgur.com/aJNupp7.jpg).

It was clear that the writers weren't sure how to proceed after having to "temporarily" write him out at the end of Season 4, and then having to do it permanently after his death between seasons. The show just never recovered. It was a sad situation all around.