A lot of information is presented at the end. In addition, if you completed the puzzle sections of the game (don't want to spoil what they are), it presents some more crazy information. Put them together and yes, the ending is a cliffhanger, but it does give lots of food for thought.
I'll spill it out in spoiler tags, so if you don't intend to play the game you can read it.
[spoiler]Basically, assuming you've played through the first game, there was a lot of mention about 'Subject 16'. The character you play, Desmond Miles, is Subject 17. Subject 16 died by spilling his blood into the weird symbols in the lab at Abstergo Industries. He discovered something, and in AC2, he encrypts and hides what he finds (possibly to protect the information from being discovered by the Templars) throughout the game. As you play Ezio, you can uncover what Subject 16 hides. The puzzles are presented here and some of them are pretty challenging. It also presents information regarding the Pieces of Eden existing throughout history of mankind and historic figures carried these pieces (from Hitler, to Jesus Christ, to Thomas Edison). If you solve all of Subject 16's puzzles, you're presented with a short video, detailing two individuals (one male, one female) running through a futuristic building holding a 'golden apple' and they refer to each other as Adam and Eve right before the video ends.
Cut to the end of the game. Ezio reaches the vault with the two Pieces of Eden he acquires. A futuristic projection appears and the character known as 'Minerva' shows up and basically explains that they are an advanced civilization and they created the human race. It can be suggested that they originally created the human race as slaves, but over time humans began to rebel, with the first sign being Adam and Eve with the 'apple', and while these superior beings (known as Gods throughout history, but in reality just more advanced) were powerful, humans had numbers on their side. War broke out, but both sides ignored the Sun. It seems that a solar flare occurs at some point during the war and almost wiped out the human race and these advanced beings. But they managed to survive and the truth begins to get skewed into myth. These superior beings are near extinction, and at the end of the conversation with Minerva, she directly addresses Desmond, which confuses Ezio.
Back in the real world, Abstergo has found out where Desmond and the assassin's have hidden and they make their escape. At the end, Lucy suggests that another solar flare, or the Sun possibly changing geomagnetically, which could cause a cataclysmic event on Earth. It's not clear how the Templars and the Assassins play into that equation though.[/spoiler]
