I stole this wholesale from a reddit post. I haven't read the story personally.

There are spoilers in this. I'll do my best to spoiler tag things, both mechanical and story, but tread carefully if you truly don't want to be spoiled on anything. This is a very long post.

• Forsaken campaign begins with the Prison Break mission which ultimately ends with Cayde's death. Petra Venj assists us during this mission.
  
• While the Scorn were once Fallen, corrupted ether and repeated reanimations has mutated them. They were locked in the deepest parts of the Prison of Elders.
  
• Mongrels tower over the battlefield, blasting out cascades of lightning that are near-impossible to dodge.
  
• Arachnid-like Screeb scramble on all fours, and explode like Cursed Thralls, only bigger.
  
• Raiders send pit spinning explosive saw blades of void energy and teleport.
  
• Lurkers carry small gladiatorial shields which block shots until hit in the leg.
  
• Ravagers (the author's favorite) hold spinning lanterns which when slammed into the ground great a spout of solar flame, unless the lanterns is shot first.
  
• Chieftains are the ones holding all the new enemy types together (much like a captain). They will throw out one of three elemental totems.
  
• Solar Totem is basically a dangerous spinning turret of flame.
  
• Void Totem gives shields to nearby enemies.
  
• Arc Totem attracts the Guardian to the Totem's center, leaving them unable to move away until it's destroyed.
  
• The Scorn are the most aggressive of Destiny 2's enemy types.
  
• The game goes to great pains to make sure you hate the guy who pulls the trigger as Uldren uses the Ace of Spades to kill Cayde.
  
• There tries to be a balance as Uldren kills Cayde trying to find his sister, and we go after Uldren for killing Cayde. It's cyclical.
  
• Each subclass gets a new tree, which drastically changes how the super works.
  
• 3 new subclass trees available on day one, then one or two weeks before you can earn another few.
  
• Forsaken's campaign takes place in the Tangled Shore.
  
• Regular Fallen clash against the Scorn, the Hive and scattered Red Legion enemies in the TS.
  
• The big Fallen captain shown in the trailer is called 'the Spider' and is the vendor for the Tangled Shore.
  
• There is an uneasy alliance between the Guardian and the Spider. His spike-backed Fallen enforcers often accompany me on subsequent adventures, lending support while rarely wading into the most dangerous confrontations.
  
• Early on, the Spider wants some new invaders dealt with, later on he cashes in more favors to demand yet more work, hunting down outlaws from the Prison of Elders from across the system.
  
• These enemies are more spread out, and can be in random world locations, strikes, in the middle of Gambit matches or in high-level Lost Sectors on old planets.
  
• The meat of the Forsaken campaign deals with the Barons in the Baron Hunts. Each Baron's name is rooted in a Western archetype.
  
• The Rider pilots a Pike.
  
• The Mad Bomber throws out explosives.
  
• Majority of Baron hunts can be completed in any order. After 6 are dead the game moves back to a more linear path as we've come to expect.
  
• You then kill 2 more Barons, then go for Uldren.
  
• Author was very impressed how every Baron Hunt felt bespoke.
  
• The Rifleman is a sniper, so the mission focuses on long distance encounters.
  
• The Trickster baits the Guardian with traps and hidden explosives.
  
• Barons are reminiscent of strike boss encounters, demanding distinct strategies to match their abilities.
  
• Impressed by the intuitive nature of the new weapon slot system.
  
• Bow is now the author's favorite weapon to shoot.
  
• New Public Event called Cryo-Cell Escape, where a Pod CELL FROM THE PRISON OF ELDERS crashes down, and enemies try to retrieve their captive leader. After a few waves the pod will open and you'll fight the leader once held in the cell.
  
• Lost Sectors in the Tangled Shore are more nuanced than before, one contains a Fallen Black Market fighting ring "and a chance to learn about the Fallen's bizarre take on a dance club".
  
• The closing act of the main campaign teases a greater threat, forcing my Guardian to turn from thoughts of revenge to something more important.
  
• A strange voice calls to Uldren from beyond the Awoken Watchtower, within which lies the greatest secrets of the Awoken.
  
• Ammo type is no longer directly tied to weapon slots.
  
• Many weapon archetypes are moving out of the Power slot.
  
• Hawthorne's Field-Forged Shotgun is now Kinetic.
  
• Primary/Secondary using White Ammo: Auto Rifles, Bows, Handcannons, Pulse Rifles, Scout Rifles, Sidearms and SMGs.
  
• Primary/Secondary using Green Ammo: Breech-Loaded Grenade Launchers, Fusion Rifles, Shotguns, Snipers and Trace Rifles.
  
• Power using Purple Ammo: Drum-Loaded Grenade Launchers, Linear Fusion Rifles, Rocket Launchers and Swords.
  
• Main Ingredient is now in the energy slot.
  
• The Dreaming City is the long-hidden home of the Awoken.
  
• Acts as an inspirational location filled with secrets, new activities, a new raid and dynamic content that changes week to week.
  
• There are story threads that you learn towards the end of the campaign that are only fully paid off once you beat the raid.
  
• Just like the Tangled Shore, the Dreaming City is a full-sized destination about the size of Nessus.
  
• You won't go to the Dreaming City during the campaign.
  
• There's a quest after the campaign to reopen access to this destination.
  
• Forsaken moves the Level Cap to 50 and the Power Cap to 600.
  
• "The Dreaming City is the most picturesque work the developers have produced."
  
• Dreaming City is infected by the Taken. After arriving for the first time you are pulled into the Ascendant Realm.
  
• Destination exists in both planes and will take weeks to unravel the mystery.
  
• There is a 3 week cycle to how the Dreaming City will appear.
  
• Petra Venj is the Vendor and guiding character for the Dreaming City, and will move around during the 3 week rotation.
  
• Over time, new missions and secrets will reveal themselves. Enemy configurations and power levels change. The Taken curse takes a greater hold.
  
• At the end of the cycle, a greater secret awaits discovery for the pinnacle players to confront.
  
• Another new Public Event in a Tower Defense style called Rift Generator. You power up an Awoken attack pillar to hold off a Scorn invasion.
  
• Loot Caches provide offerings to take to the Blind Well, a new take on Wave-Based fights like Court of Oryx, Archon's Forge and Escalation Protocol.
  
• Entire area gets shrouded in Taken Gas, except for the areas you're allowed to fight. As you work your way around you get more space to fight. Culminates in a boss.
  
• Around the DC, dropped flasks will allow players to briefly glimpse into the ascendant realm to face special challenges, or travel along platforms or tunnels that don't exist in the real world.
  
• At the center of the city is the Keep of Voices, where the new raid takes place. It will continue the narrative introduced with Uldren's madness in the campaign, and the revel of the greater threat looming behind the curtain.
  
• Bungie were surprised that Destiny 2 didn't have the same staying power as Destiny 1, so began working on new features and fixes as quick as possible, like Masterworks, Escalation Protocol, Exotic Quests etc.
  
• Does the Expansion Pass mean no more Story Content? "No, lore, story and characters are always important to experience in Destiny, the expansion pass is about moving away from the one and done nature of the story campaigns from DLC's past. It focuses on replayable experience and content that extends the game.
  
• Barrett: "We wanted to shift the tone of Destiny to be a little bit darker. We want the tone of the story and the cinematics to actually reflect that. And that doesn't mean that comedy or humor doesn't have a place in Destiny, it absolutely does, but it can't be every time a character speaks or every time the Ghost talks that they're trying to do a one-liner joke.
  
• Director has been redesigned to provide an intuitive guide to meaningful content (Flashing destinations when things are happening) and less focus on milestones.
  
• Vanguard, Crucible and Gambit tabs are at the bottom of the screen.
  
• All those activities as well as Flashpoint now refresh every 4 days, with Raid still being on a weekly reset.
  
• Individual Strikes are now on Destination maps rather than only being accessible through a playlist.
  
• One new strike goes through the Prison of Elders to dodge trains and fight on an old battlefield only familiar to original Destiny players.
  
• Ships, Sparrows, Ghost Shells, Emotes, Shaders, Emblems, Exotics and Y1 Armour and Weapons can be retrieved from the collection.
  
• Year 1 Armour & Weapons won't get random rolls.
  
• Vault is expanding to 500 slots (up from current 300).
  
• Mod overhaul features less conditions such as subclass or specific weapon requirements, but rather allow for more specialised gear, so adding a mod that increases in-air accuracy or increased damage against vehicles.
  
• Triumphs are an evolution of the Record Books from Destiny 1. Out of the gate there are sections for Character, Destinations, Events, Gambit, PvP, Vanguard, Lore and Timeworn Labours, which are Triumphs only achievable during certain time periods.
  
• New Lore, readable in-game such as Awoken Sacred Texts. 10-15 new stories to discover.
  
• We wanted to tell a story about where Variks has been.
  
• Cerberus +1 is a new Exotic Auto Rifle with 4 barrels which shoots in four directions at once, can be devastating in the right circumstances.
  
• The Drifter is not a Guardian.
  
• New PvP maps and modes won't be available immediately at Forsaken launch, will be a few weeks after. Will be 4 new maps and new mode called Breakthrough.
  
• Breakthrough involves 2 teams of 4 fighting to capture a 'breaker' which will allow that team to hack the enemy's vault. Once the Breaker is captured it turns to a attack/defend style mode with one team defending their vault while the other team tries to attack it.

Some standout things for me:

This seemingly answered Cody's concern about how the Dreaming City changes, and how players might miss something.

Dreaming City and the raid sound really freaking cool.

The new weapon system sounds neat.

Both new locations are about the size of Nessus. That's awesome!

New mod system sounds cool, but I'm not sure if they're going far enough. Separating it from subclass and specific weapon types is a good start, though.

The Spider bounties sound cool. Queen's Wrath is back!

4-day reset cycle sounds great. A week is just slightly too long, where it feels like I'm spinning my wheels for the last day or two.

Neutral Bomb is back? That's what the new PvP mode sounds like anyway.