1d4chan has summaries:
Storyline wise, 8th Edition advanced the storyline by ~250 years. Cadia fell, but the guard didn't. Abaddon turned out to be following some plan to destroy an Ancient Necron bandaid that was keeping Chaos from breaking into realspace. Once he did so, the entire fucking galaxy split in half. Everything is fucked. Everything.
In the middle of all this, the Primarchs are waking up. And the first thing Guilliman did when returning was unleash a bunch of Super Space Marines that double the entire Space Marine lineup and are completely out of place aesthetically in 40k, while the old Space Marines are all getting squatted. No other loyalist Primarchs have returned, although multiple Chaos ones have. Thanks ADB, very cool.
There was one campaign about securing a planet that is a pathway between the two sides of the giant fucking warp rift in the galaxy; another about how every faction and species in the galaxy is now becoming stupidly psychic because of the warp breaking through.
The Golden Throne is breaking, and there's no one left alive who can fix it because the Custodes never let the AdMech in to look at it. The Custodes recognize this as an oh shit tier problem and are hunting down Dark Mechanicum forge world for help.
Crunch wise, the biggest change is Command Points are no longer soup-friendly, which inadvertently made Custodes one of, if not the strongest, armies in the game. All is right in the Imperium.
9th edition steps back a bit because they realized they could set an entire Horus Heresy level of novels in the Indomitus Crusade. Rules wise, it's 8.5. The 8.0 codexes are still compatible. Timeline wise it's set around the same time as 8th edition. 9th edition changes command points to try and get rid of soup. It kinda works, but more importantly accidentally turned Custodes into the strongest army in the game. So, win win.
New game sizes. Combat Patrol (500 points), Incursion (1000), Strike Force (2000), Onslaught (3000). The new getting started boxes are "Combat Patrol" boxes, with the target being ~500 points. Imperial Knight players on suicide watch.
Plastic Sisters of Battle, because this was the compromise to the "we need Female Space Marines because the fans HATE THE IDEA and we want to fuck with them" sect. All told, the fans won that argument -- Sisters are fucking awesome. Sisters of Silence, too!
The not-main boxed set was called Indomitus, and was like 2 fuckhuge Space Marine and Necron armies at like a 70% discount. Just fucking retarded good value, even moreso if you can find it on discount -- Amazon has it for ~$170, or $30 off. Which is fucking tempting because you can almost definitely flip one half of it + the book for a good chunk of that.
Scalpers suck dick, but GW realized they can print-to-order these things and did so.
The big plot change was that while Grandpappy Smurf was running around, the Necrons are waking up because of the galaxy breaking in half. All of them, everywhere. And they're working on new anti-warp tech that just happens to completely destroy any souls that come near it. Not much has happened since then. The book team has gone back to the start of Indomitus.
