There is a veritible truck load of information the team detailing changes in philosophy as well as what they call a "starting point for a dialogue with the community." While the list feels comprehensive (though Blizzard mentions it's not meant to be), with details for every class being represented, Blizzard's goal is to give World of Warcraft players an idea of what to expect when Shadowlands rolls out later this year.
Much of this is based on player feedback it seems, with Blizzard citing players complaining that each class has started to stray too far from what made each class unique.
In Shadowlands, we want to return to the idea that a specialization is about powering up a specific part of a class’s kit, not narrowing down their toolset. In an expansion that is all about choice, we also want to give players more opportunities to make impactful choices to customize their gameplay and express their own vision of how their characters engage in combat. Many specialization-specific abilities and spells that help define the identity of a class will once again be available class-wide, and we’re reintroducing some iconic abilities to give all of Azeroth’s heroes a fighting chance against the dangers ahead.
Some of the changes include Shadow Priests getting back Surrender to Madness, with what the WoW team calls a "wicked twist." Paladins will be happy to know that Holy Power is coming back, regardless of your class spec, as well as Outlaw and Subtlety Rogues will be able to use Instant, Crippling and Numbing Poisons on their weapons, and much more.
If you're looking to get back into World of Warcraft with Shadowlands, this is definitely worth a full read to give you an idea of where your class might end up come launch day. Personally, I'm eager to jump back in with my Monk and test the changes, especially seeing Touch of Death work like it was initially intended.