A little bad News, according to what I have read on the game, it will take about 3-4 hours to beat the game once. It's a bit of a sand box, open ended thing. You're a photojournalist curious about the outbreak of zombies. In 72 hours (not realtime obviously) a helicopter will be arrived to air lift you out. You will have a wrist watch counting down time until pick-up.
If you so choose, you can sit on your ass for 3 hours and wait for your ride (this will get you a bad ending), or you can do many sidequests to gain access to other sections of the small, rescue other survivors in the mall, and investigate the cause of the Zombie outbreak. Or you can just play around killing Zombies. Doing sidequests will also upgrade your abilities (those wall kicks and martial arts you saw in the trailer). There supposedly are other enemies than zombies, there are paranoid and irrational shop keepers and thugs (a la the motorcycle gang in the Original Dawn of the Dead)
Supposedly the focus of the game will be replayability, playing over and over for fun and to get the various endings. Though who knows, things may change between now and when the game is released.