I'm going to start by saying I haven't finished this game, far from it, and I don't think that I ever will. Not because it's a bad game or that it is boring, but because its simply too much to do in it, and for a guy with a job and a family, this feels like another work, which you do not get paid for.
That being said, currently, I'm on my 15th hour of playthrough. My impressions so far:
1) Story has more quality in Odyssey than it has in Origins. This is determined by one little detail - Origins only had fully cinematic main-quests, while all of the side-quests were not. You could just "laddy-daddy-da" around while listening (most of the time I did not) to some random NPC saying almost exact same thing that his counterpart five minutes ago in another area did, and go fetch or kill someone without having any valid idea why. In Odyssey, however, this is not the case, because all of the quests are fully cinematic, so you at least know the general idea and the story behind the plight of the people you encounter.
2) Gameplay is the same, but some aspects are better in Odyssey than in Origins. For example, Origins did not have much to go on in regards of fancy fighting techniques. You just bashed your way with your sword and shield, and/or sniped your way with bows and arrows. That's it. While in Odyssey you are introduced with variety of ways (compared to Origins) to dispatch your enemies: kick them off the edge, impale them by throwing your spear, stun them with burst of light etc. In other words, the combat is more fun.
3) The world. It's on par with Origins, I think. Nothing much to see, and only on the third map where I was supposed to meet the contact, which game me my first assignment to sail from Kephallonia, I ventured in a spectacular looking valley with vibrant foliage colours. It actually was so beautiful in comparison to other places, which did not stood from the crowd, that I had to take a screenshot. But, in over-all, the world is not much different from Origins.
Also, speaking about the world, it's big, very big, bigger than Origins, I think. When Kassandra (you can choose between two persons to play for, male and female) emerged from the secret-club-meeting place, and I had the option to look at the list of possible hit targets, I just gave-up. Too many. And, when I minimized the world map, yep... too big of a world. I can't possibly finish all of this till Summer (on the other hand, maybe that's was the intention of Ubisoft, because, as far as I know, we will not receive another game next year).
Anyway, if I had to say, it's better than its predecessor Origins (compared to it, it's like a polished gem), but suffers from the same things that many open-world games suffer, i.e., bloated world and unnecessary side activities that are introduced to compensate for that bloated world.