This is a summary of the rules changes coming to Magic with Modern Horizons 2. If there should be a discrepancy between this summary and the official rules, the official rules take precedence.
New and updated rules
This is the rule that covers the planeswalker loyalty symbol. Carth the Lion uses it in an additional cost, which we haven't seen before. This rule is updated to cover that use.
Normally, only objects on the stack and battlefield have controllers. 109.4 has the list of exceptions to that rule. A new rule here clarifies that triggered abilities that have triggered but not yet been placed on the stack do have controllers. The rest of this section moved down one letter.
Previously, Clue tokens were defined by the rule for the investigate keyword action. However, a couple of cards in Modern Horizons 2 create Clue tokens without investigating, so the definition of a Clue token has been moved to the predefined token section, and now Clue is cool enough to hang with its cousins Treasure and Food. Rule 701.36, "Investigate," was updated to refer to this rule.
Characteristic-defining abilities function everywhere, even outside the game. They also function before the game begins, and this rule was given an update to clarify that fact so that you know you can have Kaheera, the Orphanguard be your companion when there are Shapeshifters with the changeling ability in your deck. Rule 604.3 is receiving the same update.
Most abilities that don't function on the battlefield state what zone they do function in. Grist, the Hunger Tide has a new kind of ability that instead states what zone it doesn't function in. A new rule tells us that this functions in every other zone, including outside the game and before the game begins. This means that Grist can be your commander but can't be played in a deck with Kaheera, the Orphanguard as your companion. The rest of the rules in this section have moved down one letter.
Nykthos Paragon has a new triggered ability with the words "do this only once each turn" at the end of it. A new rule explains that this means two things: First, it means that you cannot take the preceding action more than once in a turn. Second, it means that once you have taken that action, that ability no longer triggers this turn.
Valentin, Dean of the Vein has an interesting reflexive triggered ability. Specifically, it's one that comes from a replacement effect rather than a resolving spell or ability. This rule has been updated to include replacement effects.
Carth the Lion has a unique ability that adds an additional cost of +1 loyalty to planeswalker loyalty abilities. A new rule and some updates to old rules clarify how this works. Specifically, multiple costs to add or remove counters are combined into a single cost when determining whether you may activate a loyalty ability and how many loyalty counters to add or remove, as appropriate. This also matters for replacement effects that care about putting counters on objects.
Murktide Regent has both delve and a linked ability that refers to cards "exiled with it." These abilities are linked, and a new rule explains how that functions.
This rule, the equip [quality] creature rule, was due for a rewrite. With the way the word "creature" may or may not appear on these equip abilities, a literal reading of it was kind of weird and redundant. A lot of the words changed, but functionally, it is the same. The new phrasing did make 702.6d unnecessary, however, so it was removed and 702.6e moved up a letter.
This is the rule for protection from everything. It covers a lot of things. You might even say it covers everything. Except players, as it turns out. Teferi's Protection gives a player protection from everything, and now this rule follows suit and refers to players as well as permanents.
This is the trample section of the rules. Trample over planeswalkers ran right over this section as new rules were added and old rules were changed. Trample still works the way it did before, Trample over planeswalkers works the way you'd expect it to, and we've hopefully covered all the odd corner cases where Thrasta is attacking a planeswalker that is then (somehow) animated and blocks for itself. Don't think too hard about that one, it might create a time-travel paradox. I really don't know.
It turns out Goats will eat anything, so this new rule covers Caprichrome's devour artifact ability.
The living weapon rule has been updated to make Phyrexian Germ creature tokens. It is otherwise unchanged.
This new rule covers Garth One-Eye's ability to create copies of objects that aren't currently in the game.
Dakkon, Dihada, and Grist
New Glossary Entries
Clue Token, Trample Over Planeswalkers
Revised Glossary Entries
Specific rule references were added to Encore and Search entries.
Learn entry updated to say that it is a keyword action.