That tense moment of silence when your feet lift of the ground, the uncertainty of your next step IS Assassin's Creed.
Imagine the Mary Poppins chimney sweep scene, except for instead of Bert you have a hooded assassin named Altair, and the tawny, dust-colored brick of Jerusalem stretches out before you instead of the sooty rooftops of London. Assassin's Creed sweeps you into that lofty world in a remarkable way. Running and leaping from roof to roof, climbing impossibly high towers, and sneaking away from grounded guards who will never catch you makes for a remarkable experience--even if other parts of the game, like the combat, aren't so great. Play it for the glorious exploration; everything else is acceptably secondary.