To be fair, Cotterill was only ever probably going to be a short-term option anyway. At the time he was taken on, it seemed as though the appointment was based primarily on the fact that the club wasn't willing to shell out enough money for the more expensive compensation package that would have stemmed from the act of hiring a manager from a less financially woeful club than Portsmouth. Add that to the fact that his style of play was highly unpopular with the fans and it's not all that surprising that he's gone.
To clarify, I agree that he deserved more praise than he got for keeping Forest up and at least giving them a shred of stability in the Steve McClaren aftermath. Unfortunately for him, though, he was never likely to be seen as the man to take the club forward to the next level. Depending on who they appoint next, presumably as part of Saturday's press conference, we'll see if the owners have made a step forward or a large steps sideways with this move.