This is a quick and dirty list of all the changes from 3.5. This is also not updated fully and may not contain all the changes.

Core rules

Pounce is legal for everyone.

Old Weapon Finesse feat is free for everyone. New Weapon Finesse feat allows people to deal Dex to damage on melee weapons.

Firearm rules have been added.

Mounted characters can wield lance with one hand, but treat it as if they area wielding it with two hands for purposes of strength bonus to damage and Power Attack feat. Mounted characters can still only make one attack on their charges.

Skills have been grouped.

Crafting magic items does not require prerequisite spell.

Archon familiars can only have one of each SLA active at a time. (No spamming aid, continual lights)

Prayer beads are banned.

A character may benefit from only 1 Nightstick each day.

You cannot swap domains for devotion feats.

 

Extra turning feat is now called Devout, and increases turning damage by 1.

Two-Weapon Rend feat no longer has any BAB requirements.

ITWF and GTWF are removed.

Monkey Grip feat allows you to gain up to 10 feet reach during your turn when you are wielding a Large weapon. You cannot gain reach beyond 10ft with this feat.

Combat Form feats have been changed lower BAB.

 

Spells

All damage spells no longer have a caster level cap. Dispel magic no longer has a CL cap.

Glitterdust, solid fog, freezing fog, and rope trick spells uses the PF version. 

Cone of cold now applies slow effect for 1 round on failed save.

Fireburst, Greater now applies knockback and prone on failed save to the spell's radius.

Arc of Lightning now applies dazzled condition for 1 round on failed save.

Polymorph is banned. Alter self is disguise self spell.

Web spell is now basically monstrous spider's web throw.

Entangle's breakout DC is now the spell DC.

Fire shuriken is now an offensive spell instead of creating items.

Alarm now summons a bird that keeps lookout - this bird can be beaten with stealth.

Glibness is banned.

Chill Touch is clarified to single target.

Backbiter has been changed to transmutation.

Fist of stone is banned.

Explosive runes can't be stacked.

Reciprocal gyre only dazes for 1 round, but acts as a targeted dispel, and now deals 2d6/lv damage per spell level of the spells dispelled. (will save half)

Overwhelm spell is 5th level now.

Vitriolic sphere is 1d6/lv damage, but ref save for half. 

--Change Log - 5/31/2022

L1 spell changelog

Burning hands - 1d4 per caster level -> 3d4, +1d4 per 2 caster level beyond first.
Chill Touch - 1d6 per caster level -> 2d6, +1d6 per 2 caster level beyond first.
Energy Ray - Removed. Basically same as chromatic orbs but ray and worse.
Jet of Steam - 1d4 per caster level -> 3d4, +1d4 per 2 caster level beyond first.
Kelgore's Firebolt - 1d6 per caster level -> 3d6, +1d6 per 2 caster level beyond first.
Orb of [element], lesser -> merged to Chromatic Orb, lesser. Base damage 1d8 -> 2d8.
Light of Lunia - 1d6 per ray -> 2d6 per ray
Magic Missile - amount of missiles uncapped
Protection from [Alignment] - merged to Protection from Alignment
Shocking Grasp - 1d6 per caster level -> 3d6, +1d6 per 2 caster level beyond first.
True Casting - Removed. Celerity no longer uses spell resistance mechanic.

L3 spell

Magic Circle against [Alignment] - merged to Magic Circle against Alignment
Spell Vulnerability - Removed. Celerity no longer uses spell resistance mechanic.

L4 spell

Orb of [element] -> merged to Chromatic Orb. Base damage 1d6 -> 1d8.

General

Name change: Legion's -> Mass

 

Classes

Barbarians have powerful grip during rage.

Clerics only get one domain at level 1. They get their second one at level 3. Domains are changed.

Druids' animal companions that died now only come back after 30 days or level up. Until that happens, druids get -2 on attack and weapon damage. (same thing for wild cohort feat)

Healers now cast spontaneously off their entire list.

Hexblade has been changed accordingly to the unofficial fix.

Ninja's Ki Pool refreshes every encounter.

Paladins can serenity as ACF, but no double dipping Wis for will save. They also cast off Cha.

Rogues have special abilities at lv 5, 10, 15, and 20.

Scouts can now move at half-speed, and still full attack. Improved Skirmish has been nerfed.

Sorcerers have been given Impromptu Arcana, which allows them to spontaneously cast off the entire wizard/sorcerer spell list. (with heavy restrictions)

Swashbuckler is changed a lot. (mostly gestalted with Duelist PrC)

Wizards' Abrupt jaunt is now swift action. Wizard's Spontaneous Divination is nerfed to you must sacrifice higher spells only (instead of equal or higher).

Alienist - pseudonatural subtypes now grants 1 quickened true strike per day instead.

Arcane archer has been changed to Pathfinder version.

Assassin now casts spontaneously from the entire list, just like beguilers.

Bladesinger has been buffed.

Geometer now can be taken early, is able to cast greater glyph of warding as a 5th level spell, can cast glyph of warding without material component, and is able to cast glyph of warding/greater as a standard action if used with a spellglyph.

Kensai's signature weapon starts at +2 enchantment.

Shadowdancer's shadow jump is no longer gated by distance per day. Range remains as listed.