Yeah, there's no reasonable way for them to make the gameplay the same quality as the trailer, although that would be epic multiplied by awesome. The Assassin's Creed II gameplay was really nice though, so I don't see why this wouldn't be even better.
I also hope they sneak in a few references like they did in ACII with the "It's-a me! Mario!" thing they did. I literally LOL'd when I saw that.
I'm thinking that they might make it so that you can be any of the characters shown in the trailer, kind of like you can lock on to a certain person at the beginning with the machine-y thing. However, this would be pretty complex to make, and would also take away from the connection and continuity of Ezio.
Who knows? We'll just have to wait and see.