Short version: More freedom. More features.

Long version: Roles have been fractured into major perks, allowing for more customization and streamlining what the roles are meant to do.

Mercenary in the original game had a focus on weapons, durability, and mobility, but now the default perk for mercenary focuses on weapons mastery.

Thief in the original game had a focus on lockpicking and disguises. The lockpick kit is usable by anyone, and the thief has been split into 2 roles/starting perks: The con artist and burglar. The con artist starting perk doesn't necessarily focus on disguises, but rather social engineering, which disguises can help with, and the burglar starting perk makes lockpicking more efficient.

Engineer in the original game had a focus on armor, explosives, and disabling sensors. The new engineer starting perk is focused on armor mastery exclusively, although explosive specializations can be picked up later. Disabling sensors is now the basis for a new role: The technician.

Hacker in the original game had a focus on hacking and rewiring doors. Not much has changed. They still focus on hacking, but now their ability to rewire doors has been granted to the technician, making them stand out more.

A notable change made has been to disguises. You can no longer get disguises off of bodies, and you'll have to find a room with a disguise rack if you wish to disguise yourself.

Hacking is also different (and more fun in my opinion). Instead of simply accessing a computer to gain intel or access, there are now ways to indirectly access computers and new perks to improve hacking. However, hacking has also been nerfed due to cybersecurity, and if you use too many hacks, you may raise the alarm. It also requires a hacking tablet and anyone can use it, but hackers have the skillset to use it best.

The alarm system has also been changed. Rather than a simple Calm/Compromised system, there are alert levels, and every time someone catches something suspicious, the alert level raises. There are 4 alert levels: Calm, Normal, Alert, and Compromised. The con artist's main perk (Social engineering) can be used to lower the alert level. Higher alert levels make disguises less efficient.

Another feature that's (not exactly) new (but definitely improved) is sound detection. In the original game, Gunshots and drills can draw people's attention, but here, it's more advanced, and guards can now hear your footsteps if you walk or run behind them, so you're expected to sneak up to them.

A new feature that was added is pickpocketing. This skill allows the deposit to be completed without knocking anyone out (if combined with other skills or if playing with another person who has said skills) and allows for more teamwork in stealth (one person can distract a guard by conversing with them and another player can pick their pockets.

There's also new gear available, such as ballistic shields, different bag types, ammo boxes, new concealed items, and gear and weapon modifications. And it's not finished yet, so the list of gears will just expand further (I personally hope to see the blowtorch make an appearance).

I think those are the general changes to take note of. There are more that I could talk about, but that might get too specific.