It isn't sandbox. You travel to different cities that act as large hub worlds, with you exploring and visiting different locations within the city (bars, corporate buildings, sewer systems, etc).
The hallmark feature of the first Deus Ex, which DX3 is attempting to emulate, is the freedom of choosing how to complete objectives. For example, in the recently shown E3 demo, the player had to enter a nightclub. He pays his way in, but he could also have: -smooth-talked the bouncer into letting him in for free -Gone around back and looked for a side or back entrance -Gone in through the sewer system Once inside by any of those means, he is presented with a new objective with a similar number of solutions.
The game only goes into third-person during gameplay while the player is in cover or during cinematic takedowns.
The RPG elements come in the form of upgrading weapons and purchasing new biomechanical augmentations (the main character already has mechanical arms). The weapons are self-explanatory, but the augmentations grant many different abilities for use inside and outside of combat.
The developers have stated that they want to focus the gameplay around 4 pillars of choice:
-Combat -Stealth -Hacking -Social
You can go through the game getting into direct confrontations with a kill-all attitude, or sneak through without killing a single person. You can hack your way into a computer terminal and deactivate a complex security system, or you can sweet-talk the secretary into giving you the necessary passwords.
This is all retreading on the principle gameplay philosophies of the first game in the series (Deus Ex 1). If you ever get the chance to play the original, I highly recommend it; it will allow you to understand what the sequel is going for.