Returning after a year
Well for most things you should hit up the wiki or browse the update changes on the forums here.
For specifics, not sure exactly when you left but if you missed the end of the the first season of the Living Story the changes to Lion’s Arch will be quite a shocker. The place is quite a mess these days.
The biggest gameplay/mechanics things I can think of are the changes to traits. They’re free to change now at any time, they’ve also switched to a 1 point = 1 trait system to no more dumping 5x points per trait silliness. Along with that traits are the biggest thing to recieve balance changes and they’ve added new traits to all traitlines of every class so you’ll probably want to revisit your builds to see what’s changed and what’s new there.
Also if you switch to a non lvl 80 character or start a new alt the leveling system is completely different now. Every level up now you get a reward chest-like popup that has different things every level. Often you’ll get some free loot or gear for your character others you’ll get skill points or trait unlocks. They also usually have a tooltip informing you of some aspect of the game. The new leveling rewards are kind of annoying in that you don’t get access to traits and things as early as you used to but getting loot just for leveling up is quite nice.