The story picks up right after the previous game, Brotherhood. Some new gameplay elements have been added, such as bomb crafting, and a mini tower defense game. But the basic gameplay is pretty much more of the same, which is quite alright given how good the story is in this series. You pick up Ezio's story as he ends up in Constantinople, tracking down a bunch of keys that unlock a secret assassin library. This is supposed to be the end of Ezio's story, which spanned three titles now. This time around, we have a much older character, and you can see some details in the character animations and sound that show the wear and tear of being an assassin. You also get a glimpse into the story of the other two characters, which have yet to be expanded on in previous titles.
In the first game, you were Altiar, and you got to play through his developmental years as an assassin. In Revalations, you get to see the events after the first game through flashbacks, and you play small levels that reveal what happened with him. Its very interesting to me, because much like most people, I got bored of the first title and didn't finish it. Ubisoft listened to the critics and changed things up in the second game, which I found was way more interesting, so the repetitiveness was not as appearant and draining as it was in their first title.
At the time, the first game was a beautiful game, probably still is, and a lot of new things that weren't really done before in games. The controls were new to me at least, and was actually quite intuitive. The ability to climb almost any building was real fun, although at times I felt there were way too many gaurds on the rooftops which led to many chases which slowed you down from the main storyline.
The extra background information of Desmond is done through a bunch of side quests back on Animus island. Since he's in a coma in real life, you need to play through the game to help his mind sort things out. His quests are more platforming levels that look like a darker version of the portal games. I am not so fond of the first person perspective, having to jump across gaps between blocks is harder to gauge in that perspective, especially with no sprint or anything to help you jump further. I ended up falling a few times before I got adjusted to it. At least all this stuff seems to be optional, as you need to collect 'data fragments' that show up as gems on the map when you are playing as Ezio.
After a few hours of gameplay, I'm about 30-40% through the main story. I stopped for now, and started trying to do more of the side stories. I'll follow up once I complete the game. If you have yet to buy this game, and played the previous games, I would definitely recommend it. If you have yet to play any of the series, I would suggest you start with the second title (Assassin's Creed II), and then go play Brotherhood if you end up getting into the storyline. Then if you really like the story, go play the first in the series.