PF2 quick highlights:
Action economy. You get three actions and can spend them however you want. Attack three times? Sure! (Note: there's a -5 penalty for the second attack and a -10 penalty for the third attack on the same turn (note: some feats can mitigate this eg. one that drops them to -3 and -6 respectively)). Move three times? Yeah! Move attack move? Attack move attack? Cast a spell (typically consumes two actions) and then attack? Sweet. Got a feature on your spell where you can funnel more actions into it for a bigger effect? Very cool.
Degrees of Success: Roll more than ten below the DC? Oof, that's not just a miss, that's a miss where you also fall on your ass. Ten or more over? That's a critical! You get sweet (and clearly defined) bonus effects. Roll a natural 20 or 1? That bumps you up or down a success tier instead of being an automatic failure or success. You might just be turning a critical miss into a regular miss on a 20 (given extreme DCs) or even a regular miss into a hail mary shot, like Bard hitting that gap between Smaug's scales.
Counteract as a broad mechanic: Counterspell is now just one implementation of a greater and robust counter mechanic, wherein you make a bid and possibly get a better result. The counterspell example is that you can counter a spell of up to three spell slot levels higher than the one you spent just by rolling high (see degrees of success above). This is also how you disarm traps and dispel auras.
Counterspell itself gets way more granular. It is very different depending on which class you're pulling it from, which means it feels way more satisfying, not having been smashed into a one-size-fits-all shape. You can build it up with feats, playing with the resource economy and requirements. My personal favorite is a feat which allows you (GM's discretion) to counter spells with thematically relevant spells, like fizzling a fireball with create water. It's intricate, it's interesting, you get way more control over your kit, and you get to feel really cool when you do cool stuff. Which applies to the system on the whole.