After watching the Demo Video of the newest Assassins Creed game. The developer leading the Demo speaks of many new features present in Revelations as well as talks about and shows a section of the story in the game.  
Assassins Creed: Revelations still follows the path of Ezio, who has been the lead role in the games since Assassins Creed II, as he hunts down the secrets surrounding his ancestor Altairs' past. Although Desmond wasn't featured in the demo, i highly assume that he will be featured in the game as he relives Ezio's final chapter.   
In the demo, Ezio is shown chasing down a character named Leandros who holds one of the five keys needed to unlock Altairs' Library. In doing so, this sets up the demo to show some new gameplay and combat features within the game. 
The first thing shown when the player gains control is that Ezio can become injured and this can prevent him from free-running up and across buildings and walls. 
When Ezio is then in combat, the demo seems to show a similar combat system as seen in Brotherhood, where you can chain kills from enemy to enemy, except in the demo, their is a special killing move automatically performed on the last enemy making Ezio seem very skilled and mature then his previous self seen in brotherhood. 
When encountering enemies before combat, there are a number of new actions you can choose from; You can set up tripwire bombs over dead enemies, so when guards come to investigate, they also will be killed, you can set-up distraction bombs which help you escape from a guard chance and finally you can throw impact bombs which explode on contact with an enemy, similar to grenades.  
Since Ezio is an older, more wiser character in revelations (although still with his occasional sarcasm and witty remarks) the move known as Eagle Vision has been improved into Eagle Sense, which allows Ezio to not only tell enemies and guards apart from civilians, but also see's a small white light showing were his target has been. Allowing you too steer away from your target without losing them.  
Lastly, shown in the demo was the transition between Ezio to Altair. Ezio gets the key held by Leandros and after touching it, is shown a part of Altair's past before he became the master assassin. Then taking control of Altair, you perform a mission that happened before the events of the original game. It seems that this game will focus on both the end of Ezio's story as well as give more insight into the past of Altair. 
One the final note, the developer also briefly mentioned that multiplayer will return again in revelations with character skins based on characters from the single-player. 
So, all in all, Assassins Creed Revelations is looking to be the next great game in the Assassins creed franchise, and I cant wait to get my hands on it and play it.