I think your expectations were a little too unreal.
First thing worth mentioning, is that we've known for a long time that Assassin's Creed is meant to be a trilogy, so anybody who's been keeping up with information of course expected the ending to be a total cliff-hanger. And hey, maybe Desmond's story is a cliff-hanger, but Altair's wrapped up relatively well. After all, this first game does concentrate on Altair so much more.
Second thing is that you cannot really expect every developer to make every game 60+ hours. Even the overrated Twilight Princess is only 40-50 on average. Halo 3, as well, is incredibly short. Shorter than AC. When people describe a game as a 'next-gen experience', they're not talking about game length. That's something that's relative to each individual game, not the generation in which it is made. Some games can make it as 60+ hours, some games can't. (And guess which genre that is?)