The new "Vancouver Mulligan" rule adds the ability to essentially scry 1 if your opening hand contains fewer cards than your starting hand size. The actual rule details the procedure rather than calling it "scry 1."
The Commander mulligan rule changes in a similar fashion, although the way you get to six cards remains different than the way you do in most other formats.
The rule that talks about actions a player can take from his or her opening hand got a slight rewrite to describe "the mulligan process" rather than keeping an opening hand. This isn't a functional change. We're just being a bit clearer about cards like Leylines happening after the mulligan process, including the "scry."
Processor and Scion join the roster of creature types.
Converge and rally join the roster of ability words.
This new rule in the exile section instructs players how to track face-down cards in exile in case something asks you to choose one. The Processors in Battle for Zendikar might ask you to do this, for example.
The general philosophy is that face-down cards that were exiled at the same time and for the same reason are indistinguishable. If we went the other way, we'd just be inviting players to use sleight of hand or other manipulations to disguise the identity of the cards in question.
The way it works is basically this: If you're instructed to choose a card in exile, you can choose any specific card if it's face up or if it's face down and something allows you to look at it. If it's face down and you can't look at it, you can choose a pile of cards based on when they were exiled and what exiled them. Then a card is chosen at random from that pile.
For example, let's say you control Bottled Cloister and have five cards in your hand. At the beginning of your opponent's upkeep, you exile those cards face down. Your opponent now wants to "process" one of those cards with Cryptic Cruiser. Your opponent can't look at those face-down cards, so he or she chooses that pile and one of those cards at random is put into your graveyard. If instead your opponent can look at the cards (for example, if Grimoire Thief exiled them), he or she can choose a specific card.
Remember that once you're allowed to look at a card in exile, you can continue to look at it even if the effect allowing you to look at it no longer applies. Cards aren't exiled face down too often, so I don't expect this will come up a lot, but now there are definite answers if it does.
Subsequent rules were renumbered.
601.2 and 601.2e
We improved the rules for casting a spell in the last update, so some interactions based on how or from where you could cast a spell were clearer. For example, most players thought Prophet of Kruphix would affect a spell with bestow cast as a creature but not one cast as an Aura. Now that's true! The way it breaks down is you propose the spell you're going to cast, meaning you make choices like which half of a split card you're using, how you're going to pay any alternative or additional costs, and so on. Then the game checks to see that the spell you've proposed is legal. Then you determine and pay costs. Simple, right?
The problem was a very strict reading of the rule said that the only time the game checked anything was after you proposed a spell. You could just propose whatever you wanted and the game would eventually stop you. Well, that could work, right? Not really. For example, there was nothing (other than common sense, but who's counting?) stopping you from proposing casting the fourth card of your opponent's library. You'd put that card on the stack, find out it was illegal to cast it, and the game would rewind. Fun way to look at everything, right? So I adjusted some verbiage to account for this.
I am proud to report that this oversight affected zero actual games of Magic. I checked.
I added a sentence to this rule that talks about how targets are chosen to clarify that if a spell requires a variable number of targets, and that number is determined by the rules text, that number doesn't change after targets have been chosen. For example, once you select targets for Quarantine Field's enters-the-battlefield ability, it doesn't matter what happens to the number of isolation counters on it.
We want devoid to act as a characteristic-defining ability, but it's the first one to override information found on the card. Usually they just supply information. We'd put Maro's ability in the power/toughness box if we could fit it, but no one can read text that small. So I tweaked the description of what a CDA is.
It's now possible to scry 0, so a rule was added to clarify that this is not a scry event and anything that triggers when a player scries won't trigger for scry 0.
It's not possible for a card to have two instances of changeling, so the rule describing this case was deleted.
702.112 (112! Yikes!)
The new rules for awaken.
The new rule for devoid.
702.114 (114! That's two more than 112!)
The new rule for ingest.
The new mulligan rule: Shared Team Turns edition.
The new mulligan rule: Repetitive Commander Explanation edition
New entries: awaken, devoid, ingest
Comprehensive Rule Changes