It has recently been revealed by Ubisoft that Assassin’s Creed: Brotherhood will feature mission replays. Supposedly this is due to the overwhelming request by players, because they couldn’t replay a few key missions. Most notably, would be the Da Vinci glider mission. The reason is most likely because of the achievement located on that mission and if you don’t get it you either have to restart the game or buy the DLC.
Many people are seeing this as a way to hone their assassination skills or enjoy the game without having to continually restart.
I on the other hand see something worse. Depending on how ubisoft handles it, you could be having fun replaying missions with all your equipment. However, they could possibly take away equipment based on how early the mission is. The worse theory would be that you have all your equipment from the start and never unlock anymore, but I highly doubt this would happen.
The next bad part is that Ubisoft could have some pretty difficult mission based achievements. Imagine having to beat a mission without being spotted, or a mission where you have to do it under a certain amount of time. Some people might enjoy that, others won’t.