*SPOILERES*
Really? There wasn't a great amount of tension for me because I knew there was only one way the situations could pan out. In the first scene, for instance, did anyone really believe that the farmer would be successful in hiding the jews from Waltz? I didn't. Did anyone believe that the SS captain in the bar wouldn't get wise to the charade? I didn't. Did anyone believe they'd let the new father live after his discovery of the double agent crucial to the success of the mission? Of course not.
The only uncertainty was at the end, and still I wasn't entirely satisfied with the way it played out.
An unexpected result isn't necessary for tension to exist. One of the metaphors I use in explaining the process of writing is that the author is carefully managing the expectations of the audience. It's not possible for every possible fork in the road to contain a surprise, and in fact if you go for the "twist" every time, you risk losing the trust of the viewer. Set up some trust by having a few scenes go the usual way, then throw in a twist. If you're working up to a BIG TWIST at the end, you have to pace the buildup to it by setting up a series of these mini structures in advance.
A creator like Tarantino has both an advantage and a burden that comes with his reputation. As a lot of other people in this thread have said, part of the tension comes from the fact they know that at any moment, things can go apeshit crazy. I haven't seen the movie yet, so what I can't speak to is whether he's solely relying on his own body of work for this, or whether he's established these expectations internally within the movie (at least with the earlier scenes).
There are certain overall story expectations that even somebody like Tarantino has to be careful about messing with. A "happy" outcome of the first scene, while perhaps possible within the internal structure of that scene, would by necessity be setting up a completely different movie. What eventually happens sets up future expectations, and
how it happens further refines those expectations.
This is why a lot of thriller/suspense movies fail. They do the "unexpected" with a particular scene to throw off the audience, but never reconcile what that means to the other elements of the plot. While you're watching, it may be exciting because you never see any of these twists coming, but afterwards, none of it makes sense.
I don't know if this makes sense, but I find that it's a very useful tool in writing.