1. Combat system is completely overhauled. You hold down to block, then RB/RT for light/strong attacks, parry attacks and build up an aggression meter for a overpowered attack. You can also break shields/blocks by holding down either button to power up an attack.
2. Archery is a huge part of the combat system/stealth. They basically act as guns but with arrows. You have a sniper bow, middle of the row bow, shotgun bow and then a submachine gun bow. Seems silly but it's pretty damn cool.
3. Stealth is fun. It's pretty fluid and doesn't have the same trappings as previous games. You can sneak around without using the hide grass trope....even though it's still prevalent.
4. At any moment, you can use an eagle to tag enemies and loot. It works very similar to the Ghost Recon Wildlands drone.
5. There is a major loot system in the vain of Destiny/Division. It works well.
6. Crafting system that works like Far Cry.
7. There is main quests, and a ton of interesting side quests. They generally follow the same type of gameplay loop (ride horse to camp and eliminate somebody) but they have dedicated narrative that pushes you towards the quest.
Basically, it plays like Zelda: Breath of Wild (which plays like Far Cry/Assassins Creed). There is a interesting main quest, interesting side missions and activities that are not used to gather shit but to explore the surrounding areas of the map. It's not about ticking off a checkmark on a screen but more about seeing cool shit.
So....Assassins Creed Origins = Far Cry + Ghost Recon Wildlands + Division/Destiny and a bit of Zelda.
Yeah, it still retains it's Assassins Creed vibe that makes the series fun to play but simplifies key parts of the game. In a good way I might add.
There is an activity where you find papyrus rolls, which give you a puzzle to find a treasure. They give what area they are in and a few clues. It's like those Batman:AA puzzles where you receive a riddle and you have to take a picture of it. But you get great loot instead. They do a good job having things to do around each area so you get to explore some of it.
And it does help that the game looks fantastic....especially with HDR and 4K.
Most quests give you a reason to do so. There is always some sort of story attached to it. However, they usually involve investigating something, or clearing out a bandit camp. Some can be quite extensive in where you go.
Overall, the side quests are probably the best in the series. Not on Witcher 3 levels (even those involve a lot of the same type of mechanics...investigate area, follow tracks/smells and then eliminate whatever beast you come across) but are still pretty good. The side activities are fun in my opinion.