What does a 3.P DM need to know in switching to 5E?

[Ignimortis]

If you ever ran a game of E6 in 3.PF, then 5e should turn out to be more familiar in the general tone. It's very much a grounded system - not gritty by any means, but 3.PF is "peasant to demigod" in 20 levels, whereas 5e is more "somewhat competent mortal to very competent mortal". There are no Horsemen of the Apocalypse playable in 5e, but it does something around Lord of the Rings level well enough - Boromir did fight half a hundred orcs for some time before falling, and took out a significant chunk of them.

Also, if you have players coming over from 3.PF too, they should keep in mind that the only unique subsystem in play is magic. There are variations on it with Pact Magic of Warlocks and Ki-based casting of some Monks. Otherwise, player characters can't produce explicit effects by "pressing a button" without magic. As someone who tried to play 5e after 3.PF, this turned out to be the main reason I didn't take to the system - martial classes get too few unique tricks if they don't go for magic (Paladin/Eldritch Knight/Ranger). Expect the core-only designs of early 3e to usually work (better than they did in 3e) - Fighters and Barbarians are there to smash stuff, not do special things. I wouldn't recommend 5e to any fan of the Tome of Battle/Path of War subsystems. There isn't a good replacement for Magus or Gunslinger, either.

[PhoenixPhyre]

If you ever ran a game of E6 in 3.PF, then 5e should turn out to be more familiar in the general tone. It's very much a grounded system - not gritty by any means, but 3.PF is "peasant to demigod" in 20 levels, whereas 5e is more "somewhat competent mortal to very competent mortal". There are no Horsemen of the Apocalypse playable in 5e, but it does something around Lord of the Rings level well enough - Boromir did fight half a hundred orcs for some time before falling, and took out a significant chunk of them.

Also, if you have players coming over from 3.PF too, they should keep in mind that the only unique subsystem in play is magic. There are variations on it with Pact Magic of Warlocks and Ki-based casting of some Monks. Otherwise, player characters can't produce explicit effects by "pressing a button" without magic. As someone who tried to play 5e after 3.PF, this turned out to be the main reason I didn't take to the system - martial classes get too few unique tricks if they don't go for magic (Paladin/Eldritch Knight/Ranger). Expect the core-only designs of early 3e to usually work (better than they did in 3e) - Fighters and Barbarians are there to smash stuff, not do special things. I wouldn't recommend 5e to any fan of the Tome of Battle/Path of War subsystems. There isn't a good replacement for Magus or Gunslinger, either.

Ki is not magic (in the spells sense). And even non-4E monks can create effects that otherwise would require spells.

One ki (pun intended) distinction between 3e and 5e (and 4e especially) is that 5e does not expect you to have "buttons to press" to do most things. Class abilities and spells give you guaranteed abilities, but the baseline is competent. You don't need an explicit ability to TWF, only to do it better. Everyone can shove and grapple, some can do it better. You don't have to have training to do tasks--proficiency and class abilities help you do it better or more reliably.

That said, you're right in that currently the main "subsystem" is spell-casting. Other, lesser subsystems are
* Battlemaster's maneuvers
* Ki-fueled abilities
* sorcery points/meta-magic
* alternate uses of bardic inspiration (very limited, however)

[Ignimortis]

Ki is not magic (in the spells sense). And even non-4E monks can create effects that otherwise would require spells.

One ki (pun intended) distinction between 3e and 5e (and 4e especially) is that 5e does not expect you to have "buttons to press" to do most things. Class abilities and spells give you guaranteed abilities, but the baseline is competent. You don't need an explicit ability to TWF, only to do it better. Everyone can shove and grapple, some can do it better. You don't have to have training to do tasks--proficiency and class abilities help you do it better or more reliably.

That said, you're right in that currently the main "subsystem" is spell-casting. Other, lesser subsystems are
* Battlemaster's maneuvers
* Ki-fueled abilities
* sorcery points/meta-magic
* alternate uses of bardic inspiration (very limited, however)

To be honest, you can TWF in 3.PF without an ability, too. It's just bad. The baseline for combat maneuvers and proficiency in them in 3.PF core is "barely competent, don't try anything impractical" and it's supposed to be improved through feats and stuff. Same with grappling and shoving, etc.

Battlemaster's maneuvers are...very limited in scope. Bardic Inspiration even more so. There are some nice unique effects, but. well. Ki abilities are the same - there are very few uses for them that aren't subclass dependent, and subclasses mostly add either spell effect duplication or 1-2 unique ones. Sorcery points aren't a subsystem - those are modifications for spells, they don't give abilities by themselves, they allow you to use spells in different ways.

There's no comparison or analogue in 5e to a Veiled Moon practitioner telefragging three people in two seconds or sending someone to the Shadow Plane, a Desert Fire specialist dancing across the battlefield to make whoever they touch explode into flames, etc. I assume that was an informed decision by the designers to exclude this type of thing, but, well, some people miss that in 5e.

[Demonslayer666]

I am not in the "forget everything" boat. The similarities between 5e and 3.P are so numerous that if you ran 3e without troubles, you will not have any trouble running 5e. It's still D&D. The game is not broken by playing with feats and magic items.

Lots of stuff has major changes (fatigue, feats, Opportunity Attacks). Some stuff is gone (BAB, 5' step, negative HP, partial rounds and actions, miss chance), and some stuff is new (inspiration, advantage/disadvantage, resting, death).

My table can't get past the notion that there is no 5' step any more. After two years of playing 5e, they still do it as players and DMs.

My advice to you is try and become familiar with the new stuff before running your game. When casting a spell, read the spell description - don't go off prior knowledge. The basics are the same, a fireball is still a big ball of fire, you roll initiative to determine order, and you have to roll to hit an AC until the HP run out to defeat a monster.

Use a 5e module if you can. After running The Curse of the Crimson Throne and converting it to 5e, it's quite a chore to convert custom Pathfinder creatures to 5e. It can be done, but it required a lot of prep work each week for my game.