I played the first game when it was released and IMHO this is a massive leap forward in pretty much any category you care to name.
The first game suffered from being too repetetive, essentially you carried out the same 3-4 mission types over and over to get to the end of level assassination target. ACII is a different beast altogether. You now have a variety of missions and side missions to complete.
The controls are mostly intuitive and easy to grasp and the gameplay is excellent with a complex and entertaining storyline.
You can upgrade your weapons, armour and the town your villa is in. There are a number of different side missions: codex pages, glyphs, assassination targets, treasure and legendary assassins tombs to loot. It can be bandied about a bit too much but this game really does deserve any game of the year noms it gets.
At this price you need to buy this game.