Dan Harmon is a creative and funny asshole who is horribly difficult to work with according to himself and even his fans. He regularly would lock himself in his office on the set of Community when he was supposed to be utilizing a whole staff of writers. Even when not writing a script himself it had to be, as the cast would put it, "Harmonized" by Dan. During his first stint as the showrunner you could always count on him for a negative, but funny, comment about NBC or Sony or some executive somewhere. Luckily for Dan Harmon it's that truth he's never afraid to speak combined with a one of a kind sense of humor that makes him so lovable to so many fans.
After Dan was unceremoniously fired from Community he was soon surrounding by lamenting fans. The combination of bad mounting one of the stars, Chevy Chase, as well as a series of long standing issues with how he went about working on the show found Harmon on the outside. That one season of Community was noticeably different and soon both the show's rabid fans and the work of show star Joel McHale found Harmon back in control for the show's fifth and final season.
You can read more in depth about who exactly Dan Harmon is and why, only after weeks being back on the job, he is already a week behind schedule in this Hollywood Reporter article. If you're a fan of of either Community or Dan Harmon, or both, then it is a must read. It's surprising how this small comedy went from a well kept secret on Thursday nights to the best show to keep track of both on and off TV.