Weapon tracking has been nerfed, so that combat, and particularly player versus player, is going to be a little less easy, and ultimately, more tactical. For example, your Great Sword will swing to the last spot your reticule was targeting, even if your enemy moves. Therefore, you are going to have to predict where your enemy will be a few milliseconds ahead before striking.
Toggling has also been removed. Sunlighblade argues that this should not be a source of worry, since the game is being overhauled down to the engine, so infinite stun locking will no longer be an issue.
In contrast to the above nerfs, back steps now move more quickly, with practically 0 recovery. You will get to use them almost like back dashes, and this would presumably make it a more useful move than in Dark Souls.
On the other hand, rolling is no longer as safe as it used to be. An enemy will be able to hit even if you roll out of the way as long as you make contact. Either invincibility frames for rolling have been shortened, or they may no longer be there at all.
Hitboxes have been heavily modified. You will have to make contact with a character physically to cause damage, again in contrast to the first Dark Souls where the hitbox was wider than that.
From Software hopes there will be less problems with connectivity, now that online will be server based and no longer player to player. Lag remains a constant issue when it comes to online play, but at least PVP will be a better overall experience.
Covenants, summoning and invading are returning to Dark Souls II, very much like it was in the last game. From Software will be balancing these elements out, again in the service of better PVP.
Motion capture was used for the new game, bringing new life to the animations, as well as the general game experience. Each hand will now be able to carry as much as three weapons and/or shields. Also adding to the realism is the added weight you experience from carrying said weapons. This will all make the game more tactical.
Generally, players who tried the game demo say the game is harder, even for seasoned veterans. There are rumors the character used in the demo was deliberately nerfed, and there were other comments that the controllers in use were somehow faulty, so we may not know if this is 100 % certain.
One interesting rumoured change involves being able to walk towards and from each bonfire. If true, walking and general traveling will no longer be as tedious.
Backstabbing, a big issue in Dark Souls, has been refined in Dark Souls II, so that the pommel of your sword needs to make contact with your enemy for it to go through. This will lead to a fluidly animated three hit combo, that can also been escaped from. You can also be attacked and even staggered while attempting backstabs. Plainly put, players can no longer cheese each other with backstabs, and you will have to pick your chances.
Parrying is apparently harder to do. Demo players seem to have had difficulty attempting parrying, and there seems to be disagreement on what was particularly changed. Some say ripostes might actually what was removed. Sunlighblade hypothesizes parrying is now closer to doing a shield bash. With good timing, it will drop your enemy to the flaw, and if perfectly timed, a coffin stab animation will hit your downed enemy. Generally, it seems they want it to be a less important element now.
Story wise, Dark Souls II will be unconnected to the first Dark Souls. We don’t know yet if it is set as a prequel, or sequel, or even possibly a parallel world. They won’t share as much of the lore as they used to, so that you can fill out the gaps with your own imagination.
Graphics wise, everyone who has tried it says the lighting on the game looks really good. There should definitely be more improvements experienced in the game than less.