Regardless of what the official campaign will do, the best thing about Shadowrun Returns is going to be its toolset.
Think that the game is too combat focused? Play a user module that focuses more on dialogue. Or make your own. Some high-tier backers have already had access to the toolset and despite it having only been little over a month are already doing some amazing things with it (look up Loserific's playthrough of Opifer's Life on a Limb on youtube for an example).
Though it doesn't have multiplayer I'm getting some serious NWN vibes from it.
^This.
I have a feeling the official campaign will be decent enough but relatively short. Only reason they have include an official campaign like this is because no one is going to pay money for a toolset and the OC is essentially an example of what the editor can do.
And of course the trailer will show lots of combat because they want a trailer to be exciting: Showing lots of dialogue choices, walking around, etc. would look make the game look "boring" to the masses, as well as being potentially spoilerish. I do know from reading pretty much every article I could find about this game that there are going to be ways to play as a more stealthy character; for ex: instead of simply killing the guards, sometimes you may be able to pick up a disguise and bluff your way past them.