As far as I can tell, pathfinder 1st:pathfinder 2nd :: D&D 3.5:D&D 4
It is not the same game - It isn't a revision or an evolutionary change. It is a complete rewrite, effectively. It uses the same words all over the place, but they mean different things.
Some things are for the better. Revised, shortened skill list. Fewer spell progressions. Separated Spell List Types and Classes (there are 4 spell lists and classes get access to a list or subjection of a list -- the 4 are primal (druid like), occult (bard like, whateverthat means), divine (cleric) and arcane (wizard) -- Sorcs choose a type based on bloodline). Reduced feat taxes (kinda), dependency on classes of magic items (more but more diversified stat boosts mean fewer +stat items, theoretically; you get 4 boosts, each boost is a +2 (if <18) or +1 (if >= 18) to 4 distinct stats); magic bonuses to AC on armor apply to saving throws too). It is easy to translate from how good somene is at something and your DC to compete. (trying to find someone hiding, you dc is their modifier +10.) heighten spell is free and (5th ed esque) spell are better if cast using higher level slots
Some things are just weird (money is devalued by about 10:1, replaced race with ancestry, stat generation is not point based nor a standard array but a weird hybrid). Initiative is now a skill check, based on what the character is doing, rather than its own thing. A crit success is either a 20 or beating your dc by 10.
Some things are bad. Nearly everything is now a feat (not a problem, per se), and that's implemented oddly. You start the game with 1 ancestry feat and get more as you level; so instead of an Elf starting with a bunch of things that make them different from a Human, they start much more similar and diverge over time. A flat 1k XP per level, which puts more off a burden on the GM to address level pacing rather than level gain being an emergent property. Attacks o Opportunity are now fighter-only (not melee combat classes; fighter. welcome back casting in combat), Combat reflexes is a level 10+ fighter feat. EVERYTHING related to d20 rolls is on a single progression. your modifier is character level + stat modifier + proficiency modifier + your best of each (item, circumstance, condition) bonus + your worst of (item, circumstance, condition) penalties + all unnamed penalties. Proficiency has 5 tiers: -2 (untrained), 0 (trained), +1 (expert), +2 (master) and +3 (legendary). that's it. theoretically, the rules will limit abilities based on proficiency level, but a level 15 fighter with legendary skill with swords and a level 20 Paladin with just being trained with swords isn't going to matter. it also means dc inflation and all sorts of related issues. multiclassing is 4th ed esque, by feats only. and the feats suck.
most feats are class specific. So you can't do a rogue with combat reflexes unless you take a multiclass feat. you only get 4 general feats by level, but that's fine because there are like only 10 of them. /you do get 10 (20 for rogues) skill feats. some of whic let you do things like take 10 in a single skill. which now means literally, take a ten. no modifiers (higher proficiency raises that some; legendary means its a 30. OTOH, you need to be 15th level to have a legendary skill. many times, you'd score better with a 1 rolled. except 1 is now an auto crit failure. remember how 3.0 fixed the stupid you will fail 5% of the time, no matter what you're doing or how good you are issue? its baaaaaaaaack.)
The reason there is only 1 spell progression is because if you're not a primary casting class, you do't get spells. at all. sort of. you get 'powers' that use 'spell points' instead. Ever wanted to play a character who ddn't have to micromanage an ability pool? you get to play a fighter or a rogue. sometimes. not really.
the plethora of class specific feats makes it feel like were back to ordering character off the menu a la 4th ed. Archetypes are replaced with feat trees, and both the archetype and multiclass feats have a high feat tax.
This is not anything like what i was hoping for. 3.0 to 3.5 to pathfinder ("3.75") was a fairly clear, straight forward progression. I was hoping for something along the lines of "3.875", this took a hard left turn at Albuquerque and is heading for 4.25.beta.