If you think about it each season of Community has been really stylistically different from the others. There aren't too many shows that can say that, the only ones that come to mind are Avatar: The Last Airbender (each season exploring different parts of the world) and Earth: Final Conflict (because of the cast changes and the woeful decline in quality).
I'm interested by the change of style from Season 2 (my favourite) to Season 3. Reading Dan Harmon comments at the time he wanted to add recurring stories (e.g. the rise of Chang) and to dish out Community weirdness in little doses within episodes more than showcasing themed episodes, and this gave Season 3 a different flavour to Season 2 (even though there were some themed episodes later in the season). I can't say I enjoyed the different style of Season 3 as much as Season 2 but the change was at least interesting. As for Season 4, well the less said the better. So with that in mind: