This site's official review really didn't say much about the game, beyond the author's subjective impressions. But it has some interesting mechanics that I've rarely or never seen before that I'd like to highlight here.
1) It's tab-targeting combat in which you hardly ever use tab to target mobs.
The time to kill mobs is extremely fast. The only other MMORPG that comes to mind as having a time to kill in the same ballpark is Champions Online. So having to tab select everything would only slow you down. Rather, you face in the general direction of mobs, attack, and the game picks the nearest mob in front of you as the target. It's usually better to just blast away and just kill something than to try to be more discerning in your targets. That gives the combat a feel more akin to non-tab-targeting games like Neverwinter or Spiral Knights than a tab-target game like WoW or Guild Wars.
2) You're usually massively outnumbered by mobs much weaker than you.
This is probably related to (1), and again, Champions Online is the nearest comparable here. I've long thought that a big group of players wearing down some huge mob over the course of five minutes made you feel like much less of a hero than blazing a trail through weaker mobs. Champions Online does the latter, too, but Tree of Savior goes further. It's pretty easy to get ten mobs coming after you at once without trying to, and still kill them all without too much trouble.
3) Mobs seem to "respawn" in waves really fast until they don't.
You've probably seen some games that had quests where you have to hold out against several waves of mobs. Once you clear one wave, or perhaps just most of one wave, the next wave spawns. After a while, you've killed all of the waves of mobs and then the area is safe. But not all of the mobs spawn right at the start, as that would overwhelm you. In Tree of Savior, a large chunk of the game is like that, not just a few special quests. It first, it seems like instant respawn mobs are unmanageable, but keep killing them for a while and they eventually thin out and stop respawning for a while.
4) Mobs take a long time to aggro, so if you want to run past, you can, and safely.
If you're at one end of the map and want to run to the other, and there are a hundred mobs in your path, you can probably get all the way through without getting touched unless you stumble on terrain. That said, if you run back and forth or in circles, you can get 20 mobs coming after you, and they can kill you even if you keep moving.
5) Action combat based much more heavily on moving out of the way than aiming.
The tab target thing often eliminates the need to aim, but that doesn't mean it's safe to stand there and trade shots with mobs. Keep moving and get out of their way and you can kill most mobs without taking damage. Well, you can one-shot a lot of mobs, but I mean you can often clear a group of 10 mobs without taking damage. Try to stand there and trade shots and you can expect to get hit often and hard. The only real comparable I have for this is Spiral Knights.
6) You have enormous carry capacity right from the start
The only comparable that comes to mind here is The Chronicles of Spellborn. At the start, you can carry up to 5000 weight, and it goes up from there. Most items have 1 weight, though weapons and armors might have 50 or 100 weight or so. But you can have your storage 20% full, then go kill mobs for an hour, then have this result in being 50% full. You can do that without needing to buy bag space from the item mall.
7) Any class can use cloth, leather, or plate--but there are advantages to picking one and sticking with it
Any class can equip any armor, and the different types of armor give different types of stats. It's not like, say, WoW, where plate is better than leather, which is better than cloth. But there are bonuses to going all cloth or all leather or whatever, and it's expensive to try to get the bonuses for all of the types.