As for the blocking with a weapon and spell equiped, it's really just because of the button mechanics. In the example of a sword in the right hand and a spell in the left, the right trigger would activate the attack with the sword. The left trigger would cast the spell. If the left trigger was used for blocking with a spell equiped to it then we couldn"t ever cast the spell. That's what was meant by offensive mode. We have an attack set on each trigger.
As for the bullet time dragon shout, a previous poster was thinking of the cinematic critical shots seen in some games such as Fallout 3 which is not what this dragon shout is. The bullet time description for the shout is misleading. Like a previous poster said it's really just a slow down effect like Fables time control spell or Diablo 3's slow time spell. The game world slows down for 8 seconds allowing a tactical advantage to the player. Either the player still moves at regualr time for the advantage of more strikes or to dodge incoming attacks or the player slows down also for the advantage of targeted shots. I haven"t seen where it's been stated which way it is.
The dragon shouts are just an abilitiy like high end spells that we will gain and each has it"s own effects. The way we gain them from the walls and the way we use them seems like a nod to Brutal Legend's guitar solos, imo, which is cool because i loved that game.
I hope i helped with the confusion