Don't have time to list them all, but here is a start.
Digimon World- This game had it all, and it was truly amazing. This game didn't even rely on the anime to make an immersive game.
1. Digimon, there are a lot of them. However you can obtain 65 of them, all pretty unique. Takes a great deal of effort and planning to obtain every single one of them like a true challenge should be.
2. Battle Techniques, also a lot of them. 56 regular techniques and the 65 finishers with the digimon. Also takes a good deal of effort to obtain them all.
3. Day and Night system and Time based events. Very rare for games to have something like this in it, but it was utilized well to obtain digimon for the city.
4. Speaking of, City Building. The city will visibly grow as you obtain more digimon. Each digimon also provides something for the city in a unique way whether that be food, items, arena, etc.
5. Arena and pvp. If you have a buddy with a save file, then you can pvp them with your digimon.
6. Medals, or in other words achievements. Yes this PS1 game had specific in game achievements before almost every game out there including all of the CoDs. Was much more immersive then the 80s High Score stuff.
7. New Game Plus, when you beat the game you can continue on your old save, to finish stuff off similar to a few other games during its time.
8. Fast Travel. Forgot about this from 4, but one digimon gives you fast travel. Another rarity of its time now commonly used.
9. In Game Training. I was used to JRPGs with only battles leading to stat gains, but this game lets you gain stats outside of battle as well if you feel it worth it. Even ridiculous ways to play the slots to get 3 golden poos in a row for massive stat gains otherwise impossible.
10. Fishing. I mean the best games of all time have fishing as a minigame like Zelda, WoW, FF so why not Digimon World.
11. Card Collecting. Another side project of just opening packs and hoping to get the best cards
12. Inventory and items. There is quite a few items, I think to the order of 128 but not sure. The inventory is limited unlike most JRPGs and many items are used exclusively outside/inside combat.
13. Happiness/discipline/ Death. These things help decide which digimon you get. Be mean and let your guy die a bunch then you might have something evil. They also have different animations for when they sad/ happy so you kind of get some morality out of it.
14. Eh a few more things but I am getting tired of typing. Economy, Hilarious glitches, Trainer Rank, etc.
All these things felt pretty unique for a PS1 game, especially jampacked into a single game, and many of these things have become commonplace. Late gen sometimes brings out some real gems.