The Skill Action
In order to add a bit of new complexity without completely rewriting every class, we added a new action (the Skill Action), and detailed rules for how it interacts with some existing features (for example, all of the Rogue's Cunning Action features can work in the Skill Action, making Two-Weapon Fighting more useful). This new action also opened up the design space for feats that require active skill checks, without having to further clutter the existing actions in the game.
One of the pain points of 5e has been the survivability and performance of Monks. To help address their damage performance, we've added a feature to help them generate Ki more frequently and a better Martial Arts die progression. To aid in their survivability, we've given them more hit points and a new Ki ability called Iron Skin. Lastly, their Martial Arts feature now always counts as a Finesse weapon, allowing them to multiclass with Rogue more effectively.
Lamentably, Small Races in 5e seem to only have drawbacks. In order to capture the added evasion they enjoyed in past editions, we've given them +1 AC. We've also bumped up all 25' movement races to 30' as a general quality of life improvement.
Additionally, we made Power Attack something that Small Races could take advantage of without the penalties of using a Heavy Weapon (a Gnome wielding a Longsword with both hands can Power Attack, for example).
5e's relationship with feats has often been tenuous at best, especially when they were initially positioned as an optional rule (I guess Fighters were just supposed to max three stats?). By granting all races exactly one free feat at level 1, and making all feats grant a universal +1 ability score bump, you can now add some meaningful customization decisions at every ASI without hampering the progress of your primary attribute.
Another goal of these feats was to make Martial characters, and especially Strength based characters, more interesting.
Shape Shifting seemed like a ribbon ability for most Druid archetypes, so we've improved the base ability. We intend to write a couple more Shape Shifting focused archetypes in the future.
Noting that this feature is unpopular, we made the base rules for it a bit better, changed what the Fighting Style does, and added better feat support. The Skill Action also frees up the Bonus Action in some situations, making Two-Weapon Fighting more appealing for classes like Rogue (who would benefit from it more in past editions).
With Sorcerer, we wanted to correct their spells known and spell list a bit, since they originally only ever learned 17 leveled spells from a marginalized version of the Wizard list. We've also made some metamagic more potent. We also gave them a signature cantrip to make it more appealing to play them without dipping into Warlock.
Mostly we wanted to patch up the issues that they could run in to if the party wasn't getting enough Short Rests.
We wanted to give something unique to the wizard after we gave a lot of their toys to the Sorcerer, so we added meta composition (something that ties into their style of preparing spells ahead of times). We also gave them a signature cantrip. We may give them a special Skill Action down the road as well.