It's basically 3E but extremely streamlined (the Starter Set and the PDF have more or less the complete Rules!).
Here a few key changes:
* Bounded accuracy: Your bonuses (attack, stats,..), except HP and Damage increase very slowly. So you can use a far bigger Range of Monsters vs. your Heroes. Even basic Goblins and Orcs can be very dangerous on high level if you send enough of them and even the mightiest of Dragons can be chased away by a small army of peasants. Also magic Items are not assumed, many of them don't even have a modifier (not even most Weapons/Armor) and if there is a modifier, then it is not as high as before (+3 Sword would be an extremely rare weapon).
* Advantage/Disadvantage: Instead of Modifiers you roll 2D20 and keep the higher or lower if the the situation favors or hinders you. A simple but extremely effective change, that makes everything easier and faster for the GM and the Players.
* Concentration: Powerful Spells that have a longer Duration now often need Concentration. You cannot cast two spells with Concentration at the same time! This stops Buff's from becoming too important and Spellusers from becoming too powerful.
* Sub-Classes: Every Class gets several Subclasses to choose from at level 1 to 3 (depending on Class). This helps to make characters with the same class quite different.
* Backgrounds: There is a new layer in the character generation called "Background" which give you 2 Skills and a few other things. This is great for breaking out from the stereotype. You can play a Fighter who was a Soldier or he might been an Entertainer a Sailer, etc. You want a Ranger without Spells? Just take Barbarian or Fighter and take the Outlander Background...
* Feats are optional: Every few levels you get to choose either +2 Attribute Points (max 20) or a Feat. If you don't like feats, then just take Attributes. Feats also much bigger and stronger now, they essentially include a whole 3E Feat-Tree in one Feat. But +2 Attributepoints is very powerful so it is actually quite balanced (there are a few exceptions).
* flexible but weaker Spellcasters: Spells now use Slots, so you don't lose them when you cast a spell, you only use up one of your Spellslots/day and most Castes also have Cantrips that can be cast at will (this includes Utility Spells and weak Attacks). This makes Spellcasters more flexible (and fun) but the Spells itself are weaker and the strongest ones need Concetration, so they are fare more balanced than 1E-3E.
There are of course a great deal of other changes like the individual spells and stuff, but essentially it is still very much DnD 6 Attributes, HP, Damage Dice, +x Weapons,...) and it plays easier than ever.
