Most people would say that season 4 is the most "non-community" community season, I argue it's season 6. Personally, I think all six seasons, including the fourth, were simply amazing; but season six just felt the most different.
I think I first felt this in the second episode Lawnmower Maintenance and Postnatal Care: I suddenly realized that the Greendale seven whom we've all come to know and love has been reduced to a group of only four grown ups. To me, seven is a group, but four feels much more like two pairs of best friends - not necessarily worse, but so different.
Season six also sees a big jump in character maturity. For starters, Jeff grows a big beard and Annie looks almost completely different from her season one self. Troy's departure really took a huge toll on Abed, and I feel like his storyline was centered around finding a new "buddy," and trying to retain his childlike glee. I think the most prominent display of the "old" "childlike" Abed we see in season six is when he acts as the dungeon master inAdvanced Advanced Dungeons & Dragons, but I think that the fact that the dungeon master operates as sort of a separate entity in dungeons and dragons really indicates that Abed is isolated from the group in a way that he is the last one who still retains a bit of the child-like glee and absurdity from some of the past seasons - and he's trying his best to hold on to this.
Ian Duncan was one of my absolute favorite characters. Although he is an adult, I thought he was one of the most child-like and joyous-in-a-simple-manner characters, maybe because he just seemed happy all the time. His absence from season six, I think, really made the entire tone just that much heavier.
I think this generally heavier tone is deliberate. Harmon knows that Community is not as it was, and he knows that the absence of three main cast members is almost a sure sign for decline and eventual "death." Season six is arguable the most "meta" season, and I think this is Harmon trying to be as direct as possible with embracing the fate of the show, and I think it worked. Going through the six seasons of community was like living a life - it began with three seasons of arguably the great television ever, then a mid-life crisis hit (in the four-tees); but guess what, community rebounded spectacularly in season five, and as all things do, season six is a bitter-sweet ending to a now fully matured being.
Regardless of this, I would still sell my left arm to see the movie - everyone deserves an afterlife!
Sorry about these incoherent ramblings. Felt a little down after finishing another watch-through, but I just decided to start over with season one yesterday, and that did the trick for me!