The Nioh 2 alpha test was pretty great. It's a little better graphically than the first game, but not by much. I don't care about graphics, so that point is kinda moot for me. Enemy AI seems to be a little better, and there's a lot of enemy types that were carried over (giant yokai, human enemies) while there's also a bunch of new enemies in both yokai and human forms. The enemies in the first level killed me quite a lot, and that's after playing the first game for 300-400 hours. Difficulty was on point, I didn't manage to kill the first boss.
The new yokai transformation is pretty sweet, I'm not sure if that takes the place of the Living Weapon from the first game, or if you get to use both. You do still have the spirit guardian, so maybe you can do both eventually. There's also yokai cores that you can get as a drop from yokai that give you a special skill that uses a new resource bar. Yokai cores are unidentified when you pick them up, and until you identify them at a shrine (which happens automatically when using a shrine) they are dropped on death and held at your grave, along with your spirit guardian and amrita. The cores I got were short summons of other yokai, or a quick transformation into a giant yokai for one big swipe. Pretty cool addition.
Returning weapons are the odachi, spear, katana and a new weapon is the dual hatchet. The hatchet gives you a ranged option, throwing the hatchet at enemies. The other 3 weapons are pretty much the same as the first game. Daddy always said don't fix what ain't broke. I might miss the giant axe and the kusarigama, but I think the 4 weapon types are better. Easier to balance and keep a unique style for each, and maybe DLC will add the others.
Leveling works the same. Gather amrita, pray at a shrine, use amrita to level up. Much of the menus and inventory are the same, which is ok because the menu worked pretty well. Maybe it could be more streamlined, but I have no idea how. You get metric tons of loot to sort through, which can seem overwhelming until you realize there are filters to allow you to only pick up certain rarities, or to sell anything from a certain rarity or lower.
I read that the blacksmith, tea house and a few other features return as well, which means that there will likely be the same faction war kind of thing going on, along with PvP. Reforging armor and obtaining blueprints will probably be the same as well.
As for summoning / multiplayer, the red Bloody Graves return, which you can use to summon a revanant based on the person who died there. Certain developer placed graves also hint at hazards on the area, or the wall-yokai and how to appease them with gestures. Also added are a new, blue grave which is placed with a certain consumable. It serves as a helper revenant, so someone else can use it and get an AI helper based on your character. This helper also gets you rewards if people use it, which is pretty cool. You can still summon and search for others to help in the same way as the first game.
The biggest change that I saw was the way the skill trees are organized. They look more like the ones in Path of Exile, being a circular tree pattern. Most of the skills I saw are the same, and some like the Iai Quickdraw have been nerfed heavily to bring them in line with other skills, while others like some of the parries have been buffed slightly. Samurai skill points still come from leveling and locks of hair, and have their own dedicated tree to be used in. Each weapon also has it's own tree, and those points come from using a weapon and gaining proficiency with it, and locks of hair. Onmyo and ninjutsu are the same way, with their own tree and locks of hair.
I tried the summoning a bit, and found that it works as good or better than the first game. The first level was a little on the smaller side, compared to the first level in Nioh, but I think that's ok because the first level in Nioh was a hell of an obstacle to overcome. It did have many different paths to go through, and lots of hidden nooks and crannies with loot and new yokai. There's even certain areas that are totally contaminated by the yokai presence, and have a pretty cool effect on the area until dispelled.
I think I've rambled on enough, so for a TL;DR I'd say this: Nioh 2 takes what the first game did well, and makes it better. If you loved the first Nioh, and your main complaints about Nioh came down to "enemy variety could be better" and "streamline some QoL stuff" Nioh 2 is what you want. It's pretty freaking awesome.