Well here's some brief gameplay footage:
http://www.rockpapershotgun.com/2013/06/14/theres-thief-4-gameplay-footage-too/While it's never wise to judge a game based on 2 minutes of gameplay, I find it telling that this is what the developers chose to show.
The question was even put to the dev "So what makes you a Thief in this?" The devs response amounted to "Oh, this sneaking around bad guys and taking them out, feeling really powerful."
They say their feedback was that players wanted to be able to defend themselves when they fuck up, instead of "it being game over if you're seen by a guard." Really? Game over huh? I guess the pile of dead guards I'd create when that happened was not me actually playing the game. So now combat is set up for you to kill bunches of guys like a ninja, because that's totally being able to defend yourself when you make a mistake.
They say they feel like they've found a good balance between action and stealth. What does that look like?
How about:
-Focus showing you critical spots on an enemy you can strike in melee to stumble or stun them.
-Focus allowing you to point blank shoot guys in the face with a bow.
-Focus letting you do headshots to guys at distance by increasing your aim.
Is there any experience system that rewards combat badassery? You bet! Hey, anyone remember Deus Ex, where the XP system "encouraged" you to play non-lethal, and take out every single enemy in the level, because it's what granted the most xp and let you get the most mods? Yeah. You know what I didn't see? Getting xp for being sneaky.
Shadow seems important, there's Water Arrows again and dynamic lighting. They say the typical "the game is about playing your way and it adapts to your play style", implying the game is just as interesting and engrossing when played stealth as when played aggressively.
Again, to me it's about what the devs chose to show. And what they chose to show was a mostly combat-focused, assassin-style play style, where there's new specific features dedicated to combat badassery. Stealth gets a token nod in the playthrough, and is talked up, but is not shown with the same emphasis. Instead of showing, perhaps, one of the open areas of the game, they opted for what seemed to be more of a corridor level. Not enough was shown to really make that determination with surety.
So yeah. It looks like Dishonored, frankly, and its feature set seems to share that same mentality of "stealth and combat are equally important." It's hard to know what the actual thief aspects of the game are though because they're not showing them and they completely ducked the question when asked. I expect a lot of unavoidable combat in the game, because it seems to me "a whole game you can stealth through wouldn't work today" is the kind of thing a suit would say.