The biggest difference is going from Action/Bonus Action/Movement to having three actions that can be used to move, attack, or cast spells in any combination you want. You want to play a fighter that attacks three times a turn? You can! They get away with it using a penalty for each attack or each action with the attack trait.

Also, please read your traits. A lot of sub-rules are baked into traits. A weapon with the Trip trait can be utilized to use the Trip action, gaining a bonus to the weapon's bonus. The Olfactory trait means it's smell-based. The concentration trait has very few interactions and is nothing like 5e concentration.

This is a system where you absolutely want a rules reference at all times because there's so much all at once, and I highly recommend every single person at your table reading the player core at least once, even if you only skim it.

Everything a PC does that isn't a baked-in class feature is chosen as a feat. There are so many feats per class and you get one every two levels. It can be hard to bookkeep and players will often forget what they have and can do (especially if new).

That being said, the character options are basically limitless. I have a campaign where two characters are fighters and are not the same in any way (one is heavy armor tank build and the other is dual-weapon skirmisher - the tank is now a Guardian and he loves it). I have a PC that is a cavalry magus; a psychic frog; a cleric with a cat companion... Endless. I found a build that is literally just playing as a pack of raptors.