What are the Comprehensive Rules?
Magic is complicated. No, really. When you have more than 14,000 interchangeable game pieces, you get some freaky interactions. The Comprehensive Rules cover everything the game has ever come up with, from basic game play structure, to every keyword ever, to entire pages dedicated to single bizarre cards (hello, Karn Liberated!). The Comprehensive Rules are, well, comprehensive…but they're also obtuse, unfriendly, and looooong. They're not intended to be a player resource—they're a judge resource, a rules guru resource, and a place to store definitive answers. In fact, I honestly recommend never reading them.
For a much friendlier rulebook that is intended to be a player resource, check out the Rules Page and download the Basic Rulebook (4.37 MB PDF). It doesn't have sections about phasing or subgames…but you'll never miss them.
This new rule clarifies what you pay if you are instructed to pay an object's mana cost that includes X at times you aren't casting that object as a spell. This comes in handy for manifest, but also for past cards like Disruption Aura. Essentially, if the object is a spell on the stack, you pay the actual value of X. Any other time, X is 0. So you could pay GG to turn a manifested Hooded Hydra face up, netting five +1/+1 counters because of the last ability. Sweet deal.
Conspiracy finally joins this list of card types. Strange that it was absent so long. I wonder if it meant anything.
We bid a fond farewell to Anteater as a creature type.
This rule is located in the list of different kinds of linked abilities. This new addition covers the "Siege" cycle from Fate Reforged we'll talk about below.
Hey look, it's below. The Sieges are a cycle of enchantments that offer a choice between two abilities as they enter the battlefield. While on the battlefield, a Siege will have the chosen ability. These abilities are preceded by anchor words (namely, "Khans" and "Dragons"), that give the abilities a bit of flavor and signify your choice. These rules give them the structure to do the job.
Some minor language tweaks were made.
The new rules for bolster.
The new rules for manifest.
This rule covers Auras that both give the enchanted creature protection and state that "this effect doesn't remove" either that specific Aura or all Auras. In the case of Spectra Ward, it wasn't totally clear that other Auras could legally enchant the creature in the first place.
The new rules for dash.
Some minor changes to this rule that tells you what you need to track about face-down creatures you control. In addition to making it clear the order in which face-down creatures you control entered the battlefield, you must also know what ability or rules caused the creatures to be face-down. In practice, this usually means make sure you know which face-down creatures are manifested and which ones were cast face down using the morph ability. But hey, Ixidron and Illusionary Mask could be getting in on the fun as well.
The rules for double-faced cards didn't tell you how to determine their characteristics when outside the game. You look at the front face. This is similar to what you do to determine the characteristics of a double-faced card not on the battlefield.
This rule stated that double-faced cards could never be face down. Well, manifest blew that all to pieces. New subrules detail how a double-faced card behaves when it's put onto the battlefield face down. Namely, it can't transform and it will be front face up if it ever turns face up.
This rule previously explained how to exile a checklist face down. This information was moved to the new 711.6 subrules.
This rule gives underlying structure to Planeswalkers that can be your commander.
The fact that you also consider the back faces of double-faced cards for Commander deck-building legality purposes is an exception to rule 711.2a, the aforementioned rule that helps you determine the characteristics of double-faced cards. This exception is now explicitly called out.
I changed a word in the description of the Archenemy variant. You might be amused at which one I changed and why. I won't spoil it.
New glossary terms: anchor word, bolster, dash, manifest
Comprehensive Rulebook Changes