This one’s pretty straightforward, actually!
Weapon Mastery class features are an all-new property introduced in '24. These shouldn’t (but easily could) be confused with the similarly-named “Weapon Master” feat, which allows use of a Weapon Mastery property independent of character class.
The “martial” non- and half-caster classes get access to Weapon Mastery properties of two weapon types (except Fighter, they get three) at level 1: Barbarian, Fighter, Paladin, Ranger, and Rogue.
The Mastery Properties apply to all weapons of a type - Slow for longbow, Topple for quarterstaffs, etc. - regardless of whether they are magical or altered in some way.
Chapter 6: Equipment contains the definitions of each of the properties (p. 214); effects are unique to each property and can add quite a bit of tactical power to a combat. The Topple property, for instance, can impose a Prone condition on the target that could, if applied early in a round, be a substantial boost to the party’s ability to dispatch a foe.
Personally, I’m really interested in seeing how these play long-term and really enjoyed leveraging Slow from my Ranger’s longbow to keep well out of the reach of melee combatants while hammering them with arrows and Hunter’s Mark - Colossus Slayer helped wheedle down the boss in that fight, too.
I have a criticism of how these properties are presented in PHB24 and the D&D Beyond app (as of 2025-01-21T21:58:00Z)
PHB24 adds the name of the Mastery property to each weapon item within its pages. Beyond also added the property to every weapon item, and did so without indicating that this is a new game mechanism for 2024.
I understand the desire not to mark every weapon as “Legacy” and add a duplicate-text version of it that includes the new property - but those two steps weren’t necessary at all!
The specific property is determined by the type of weapon - every staff has Topple, every longbow has Slow, etc. So the Mastery Property is inherited from the weapon type, and is only available based on specific character Feats (from either class or player choice).
In its simplest form, only 5 classes are able to access a Weapon Mastery Property, and they choose which one based on the weapon’s item type.
This should have been a single table listed in the Glossary and referenced by players when selecting and adding these values to their character sheet.
I’ve seen several lines of discussion around this feature claiming that this is going to be a (further) drag on combat and make it (even) slower.
That’s possible, because enemies will now be rolling saving throws to avoid many of the effects of these properties, but I don’t think that’s the whole story.
First of all, my combats have only extremely rarely been slowed down by the weapons-oriented classes: Barbarian, Fighter, Paladin, Ranger, and Rogue are so straight-forward to play that they practically come with an instruction sheet of “stand here, hit (smite) this.” Giving them some debuffs to toss out on a hit isn’t likely to add minutes to a combat (caveat: once we’re reasonably comfortable and mature employing those effects).
Now what has slowed down combat, and usually does, is the spellcasters either a) not having a plan of action b) not understanding how their spell works c) dog-piling multiple chained effects (ahem Sorcerer cough) or d) any combination of the above.
Thing is, people gravitate toward playing spellcasters because they’re interesting, tactically-chewy classes. Look at how much power they have! Fireball with bed-head and morning breath is waaayy sexier than a dressed-to-the-nines Extra Attack. Elemental flexing to grapple a target 15 feet away gets the W against +2 to damage and resistance to non-magical damage.
If a GM wants to gripe that this is slowing down their combats, then they need to just write out the effects on a sticky note and slap it on their reference screen - because the problem isn’t the feature, it’s the GM.