Just about everything. I'm not sure what they didn't change at least a little. They went through and rebuilt the nWoD with an eye to make every choice feel good and add something both useful and interesting. Vices and virtues don't have a list of presets any more; combat is smoother and far less exploitable (no more multiple actions); social intrigue has its own full system that is NOT just a reskin of combat, a process of push-pull influence that can be very gradual. Every Merit from Barfly to Fighting Styles to Mentor has been updated to feel like a viable and important part of the character. Morality isn't a sanity-evil meter.
Vampires feel more powerful, more dangerous, and most of all more iconic - you are not a walking corpse with superpowers, you are a vampire, and you do vampire things. Every vampire can see in the dark, spot or scent blood in tiny quantities, and hear heartbeats - the stronger you are, the clearer your senses, and an elder can tell you what floor a mortal is hiding on from the bottom of a building by the furious hammering of its terrified heart. Tasting blood gives you almost as much information as a medical battery - as well as other insights, if the blood you've tasted is Kindred. They treat all mortal weapons as bashing, save of course fire. They also treat all aggravated weapons as lethal, save again fire and the sun and certain very rare powers - Protean isn't enough any more, for instance, though it will let you deal lethal damage to vampires.
Every Discipline is potent and adds new options at every dot, with the exception of the Physical Disciplines that all include active and passive benefits instead. They have the same shape - Protean is still about shapechanging - but different functions. The first dot of Protean lets you sink into earth, stone, or concrete and drink blood spilled where you rest; the second grants you a list of animal traits; the third lets you take the forms of predators you kill yourself (limited by size and by the predator/plague-bearer/parasite/carrion-eater type restriction, but otherwise whatever you hunt is yours); the fourth adds the traits of monsters and, yes, you can manifest wings, webbed claws, or blood-drinking sharkskin whether you're human-faced or a spitting cobra. With the fifth dot, you still turn into mist, only now you can drink from open wounds or asphyxiate hapless mortals. Every Discipline got a similar treatment. Most Disciplines are more powerful and provide better options earlier - you can actually order someone around effectively from Dominate 1, for instance, rather than waiting until 2 - but Vampire was always the least "potent" line in that regard and overall the game feels more intense rather than just scaled-up: a Kindred with powers you don't know is scary.
Predator's Taint is gone. Vampires still recognize each other immediately, but they don't provoke strong reactions without choosing to lash out in one of three auras of the Beast - the fight-or-flight (Strength), the mindless indulgence (Presence), or the dominate-or-submit (Intelligence) depending on their intentions. This evokes a contest between themselves and their target, winner takes all - the other Kindred chooses their own form of lashing out, and if they win their Beast overtakes yours. It costs Willpower, but you can lash out at mortals for free.
Embrace costs a dot of Humanity, not of Willpower. Humanity works differently, with the need to maintain Touchstones - human vulnerabilities that can be discovered and exploited - and develop Banes should the Beast begin to overtake you. Cruac and Theban Sorcery have been refined (although Blood Sorcery is notably still supported as a replacement); Coils of the Dragon are awesome where once they were only conceptually awesome, and include unique Devotions called Scales if you want to get really freaky.
Lots of other stuff, like the Strix themselves - hungry darkness, vampires that were never human and consider the Kindred to be weak, pointless, and perverse, in desperate need of teaching - or making into lessons, corpse-possessing demons and relentless in their taste for pain, death, and the ruin of the world. They especially hate Humanity and any Kindred above the level of draugr they see as an affront to vampirism, often one in need of correction one way or another. They're not neat and clean - in their natural forms as owl shadows they drink breath, but when they wear corpses they are more likely to consume flesh directly. They don't care about Kindred courts or the Masquerade: when they come, they steal life and tear apart whoever and whatever they please and leave the streets drenched in mortal blood, jumping corpse to corpse at whim, laughing at the terror of their once-human cousins. If the Kindred are lucky. God or Crone help a Court that has to face even one Strix strong enough to take vampire corpses, which, yes, some of them can do. There may be no help at all for the more terrible truth that however strong they are, many Strix have all the intelligence and patience to stalk and destroy Kindred society from within or without as they prefer. And all of them have the desire. Rather than just the standard toolkit approach there's actually 20 unique Strix to use, if you're interested in ST tools, and of course the full make-your-own guide is there as always.