aesthetics
Skill trees are back and so are Rune Words, but they probably wont be as intricate as D2. Its sill very early and they said its not coming out soon or even blizzard soon so its subject to change, but I saw a skill tree with branches on a stone background. Their goal was to bring back the D2 aesthetics. After the opening ceremony I had to rely on streamer's that bought the eticket and wade through their blabber to hear what the panel was saying. There was a bit more gameplay shown there. But to answer your question, the combat will def be similar to D3.
A few notes from memory
It will be open world with 5 regions, hundreds of legendaries, and 5 classes to start. I can only speculate that Necro is one of the sure fire unannounced classes. Paladin is a classic, but I could see Demon Hunter returning to be the 2nd ranged and the physical ranged at that. I don't think Monk and Witch Doctor will make the cut.
Druid can transform on the fly, chaining mutli form attacks. Barbarian is straightfoward bruit force as always, I didn't get to hear the Sorceress part because it was muted as the streamer wanted to talk. 🙄 EDIT: I'm def going Barb. Watching the gameplay trailer again I notice he can swap between 3 different weapons in combat. The first he shows up he opens with staff, swaps to sword, and then to dual axe, holy shit. Hes got a maul on his back in a lot of clips while he fights with other weapons.
Rune Words are back and they will shift away from heavily focusing on gear sets, and place a lot more emphasis legendary gear. They don't want to pigeon hole people into builds anymore, and want you to be able to create a build you envision. Sets will still be a part of the game, they just won't be better than everything.
PvP will be in the game in public arenas, World events with world bosses will be in public areas as well. They showed 7 people taking on a big monster that is near the end of the gameplay trailer.
Mounts are customizable (probably the monetization of the game as leaked). Right now they're just horses, but you can change out their armor, skin, have trophies like demon heads and what not that hang from the back. Each class will have dismount skills. They showed the Sorceress turning into an ice ball and freezing a trash mob while the barbarian followed up with a dismount ground smash turning them into dust.
Dungeons will be 100% fully randomized and the story will be non linear. You can go at your own pace, explore the world, do repeatable events and jump back in at any time.
Thats all I can remember, but I believe that was most of it. They have another panel tomorrow about the game's systems.
EDIT: here is a pic of the skill tree briefly shown.... although now looking at it its a talent tree, the skills are a different tab.