An upgrade on almost every point from the first game. Not only do you get to play two completely different scenarios with each ones having exclusive areas/bosses/weapons, the order in which you play them will make your run slightly different, because events of the first run will have influence on the second one, which is really cool.
There's more monster variety, and more monsters overall: while RE2 definitely gives you more resources than RE1, a regular zombie takes on average more gun ammo to kill, and there are way more zombies attacking you at once this time around.
The game also has its story more involved with the progression. While i do like how RE1's story is mostly just you figuring out some plot elements from notes you find, this time around you have more characters being involved, most notably the final boss and biggest threat of the game is a recurring fight and it rules (his design is absolutely awesome too)
The only thing i really miss from RE1 is the exploration, once you exit the police station you have way less puzzles and exploration to go through, which is an element of 1 i really liked; i don't mind it that much since the first part of the game is still pretty "metroidvaniaesque", i just wish we'd get more of it

Also Mr X is a huge joke in this game he's just a scripted event that does chip damage it's hilarious