The engine's workings and the way the art assets are made suggest to me that the game won't really try to use large open spaces for the actual gameplay.
Yes they've shown the level from far away while the shuttle is landing, but no, you can't just get out and walk there. The buildings are empty, and it'll stream in the parts of the level on a pre-defined path. Even different floors of the same building have to be loaded separately, so you can't really interact with anyone or anything there.
Remember, their (on-screen) texture budget is significantly smaller than for example Halo3's; they have less memory from the start, and traded in a lot of it for deferred rendering. BR may provide huge levels but you'll never interact with more than a small part of it as it's streamed in and out of memory.
So it's a clever illusion of freedom, covering a relatively linear path. There's absolutely no problem with this and it can make for some cool gameplay as well. Remember Half-Life 2, it was pretty much the same, but with similar loading pauses instead of the seamless streaming they've promised for the final version of KZ2.
Bungie on the other hand has decided to go for a lower level of detail (the comparisions obvoiusly favor KZ2) but they have huge open spaces that the player can explore, and it takes a lot of time even on vehicles. And they've promised (though we haven't seen it yet) a battle with as many enemies and allies as the entire first level of Halo 1. I just can't see KZ2 matching that... but it won't have to, either, as the two dev teams are clearly making two different games.