Played it a bit.
-Dumb AI is still dumb. Even the enemies that are supposed to be smart still accidentally suicide off cliffs.
-Highly recommend using firearms as much as possible. Early enemies are weak to them, and bullets really aren't that scarce. You can buy them in shops and find a ton off bodies.
-The transforming weapons mechanic is pretty cool, but the novelty fades pretty fast when you find that a few weapons have crappy attack animations.
-New dodge mechanics are nice.
-Lots of animation reuse from the Dark Souls games. Sawblade copies a lot of the longsword animations, as does the kirkhammer.
-You're beefier in the early game compared to Dark Souls. Boosting defense does seem to have some effect this time around. Note, however I played a copy that does not have the day 1 patch so I don't know it this'll change on launch.
-Shard items used for upgrading weapons are plentiful. Very plentiful, in fact, so you don't have the horde them and focus on one weapon anymore. You can upgrade all your weapons and experiment and try out the ones you like the best. This definitely encourages more experimentation and subtly discourages hording.
-You'll be fighting crowds of enemies (at least early on) compared to Dark Souls which focused largely on one-on-one encounters. Most early trick weapons and firearms have crowd-control attacks for dealing with these larger hordes. There are still larger singular enemies, and they're as beefy and as dangerous as ever, luckily they don't punish you as severely as dark souls when you mistime a dodge.
-Backstabs and Ripostes are gone, replaced with the new firearm mechanics. Firing your gun takes a certain number of Quicksilver bullets (1-3 so far) and can stagger weaker or low defense monsters. You then rush in with a melee attack and do a powerful critical hit on the staggered enemy. Even bigger enemies can fall to this. I don't quite know what stat triggers the possibility of stagger on tougher monsters/high defense monsters, but you can stagger them with your gun too. Just takes some patience and effort.
-No magic as far as I got, but lots of useful items (throwing knives, enchantment tags, molotov cocktails, etc). Not quite as overpowering in the early game as they were in Dark Souls, though.
That's all I got. I don't have the game, and I doubt I'll be able to play it again anytime soon, so there you go. As an FYI, I don't know what the secret mechanic is, but I'm guessing it might be some kind of vampire form. Lots of subtle Van Helsing references.