I think I've seen and read just about every preview so far. It really seems like they're building on the strong foundation HR has in a lot of areas. HR always had a strong art direction, but the tech was weak. Its not mindblowing now,
but it much stronger across the board from the 2011 release. While we haven't seen the social hubs yet, the level design in the combat zones seem stronger, larger but densely packed with several routes and increased verticality. More lively lookin with the new engine. The previews mentioned how the Prague theater was part of a larger hub, instead of some separate area you fly to in-between cutscenes/load times, so it feels like its part of the world and not just another level.
Controls are generally slicker. The UI more intuitive. Combat isn't total garbage, a controversial change for many Deus Ex fans I'm sure. The retinkering of the Aug systems encouraging more improvisation and experimentation in your playstyle. There's a flexibility, a grey area between stealth and combat you can dynamically gauge based on your tools, the level, the enemies, and the objectives. Its a much smoother, more fluid game, right down to the takedown animations no longer having that clunky screen fade before awkwardly resetting you.
The side quests sound interesting, conceptually. Maybe they're important to Jenson personally, or reflect the core themes of the game in an interesting way, or shed light on some of the factions operating in the world. They'll feel like main missions, instead of wandering around to find 3 Buffalo Butts. There are also little side objectives like "Don't kill the undercover agent" in the tutorial mission. If you kill him, some of ya buddies call you an asshole, and maybe that will change the game in a major way. Or maybe a small way. Or maybe not at all. They don't make everything have obvious GIANT RAMIFICATIONS so you're pretty much forced to do them, but they're not all meaningless either so you can't be bothered. So you can just play the game, make your choices, and live with them. There's no GOOD or EVIL options or meters. Choices you make in the first few hours of the game will impact the end. The choices you make that affect the narrative won't just be in the big obvious conversations or cutscenes, but in the moment-to-moment gameplay as well.
The last game's soundtrack had three phases: ambient, stress/proximity, and combat. Mankind Divided has a wider range that dynamically changes based on AI states(suspicious, alarmed) and proximity, and the combat music when there's a dozen guys is much bigger than when there's just two dudes left to hunt down. These kind of audio cues gives the player more information about their surroundings, and aids further in immersion within the game.
They know everybody hated how restrictive the bosses were in HR. If you went all in on hacking and stealth(and since combat was ass in the game, you did), you felt useless against the bosses. Now they'll be more options no matter what path you decide to go down, even non-lethal ones. Which is how a boss fight should be, testing your knowledge of the abilities and options at your disposal.
So they got a lot of really sound parts here. If they can put all those together with the conversations/social hubs/atmosphere/world-building and characterization fans expect from Deus Ex, it could really be a hell of a game.