The combat is tight and strategic- button mashing will get you killed. There aren't button combos or QTEs, you are directly responsible for each and every swing of your weapon. On the PS3 controller, R1 is attack, R2 is stronger attack. You can press triangle to wield your weapon two-handed, making even stronger R1 and R2 attacks respectively. For some weapons this gives 3-4 different attacks, for others '2-ish' with variations. Kind of like in SotN, you don't know how a weapon behaves until you play around with it. Some have more diversity than others. Some are plain and simple, some have magical properties.
Rolling and doing an attack right after, or nudging forward while tapping attack gets you some more attacks and moves outside of normal R1 and R2 button presses.
The equipment menu feels like SotN. You can equip different pieces of armor for each body part, you have two ring slots, and you also have two hands. You can put a sword in one hand and a shield in the other if you'd like. Typically the right hand is meant to be offensive and the left hand defensive, but you can still mix and match weapons for different reasons.
You can have backups for both your left and right hands, meaning you're only holding 2 things at once but have 4 things equipped. Pressing left or right on the d-pad switches out your left and right hand weapons, respectively (so you can keep a shield in the left hand but swap between a sword and a whip in the other by tapping the right directional button).
The right thumbstick is camera control. You're always in control of the camera and it never shows you where to go. You have to pay attention and look around on your own. Exploration is heavily rewarded- there are items littered throughout the world. But the risks are also high- run blindly into a room you're unfamiliar with and you're risking a high chance of death.
These games are known for their challenge, and yes, they're quite challenging. But this series' bark is worse than its bite. Similar to something like Mega Man 9 or Super Meat Boy, you are going to die A LOT in the process of learning the game. This doesn't mean the game is brutally impossible or anything like that. Rather, it feels like a game with infinite lives. There's never a game over, you don't lose when you die. You're undead, death is part of your character's routine- it's no big deal. :)
It's often stressed how much you lose when you die in Demon's Souls and Dark Souls, but I can tell you that it's greatly exaggerated. When you die, you go back to the beginning of the level (Demon's Souls) or to the most recent bonfire (Dark Souls), and all enemies respawn. You also lose all of your souls, which double as currency for items and XP/ leveling up. This sounds really brutal from an outside perspective, but to be perfectly honest, getting sent back to a bonfire is often less overwhelming than getting sent back to the beginning of some levels in CV3. There's little reason to horde souls because there aren't lots of super expensive items, so losing souls isn't as big of a deal as it might seem as well.
All items you collect and all shortcuts you open are permanent. There are some one-time enemies and minibosses as well. So when people talk about how brutal these games are, I feel like they're neglecting to mention how it's easy to make quick and obvious progress. It may take you a while to beat a level, but your goal should be to find things and open doors first anyways. Dying in these games is almost more like dying in a Zelda game- you get sent back somewhere and all the enemies are back, but at least you get to keep all the cool shit you found in the dungeon. If you clear out just one item from a level, you're advancing in this game.
Compared to the GoW format that LoS had, I feel like Dark Souls does everything right. It's how a Castlevania should be. Instead of arena-style battling, you have a specific, countable number of enemies that always appear in the same part of the map. This makes it feel much more like a 3D version of an NES Castlevania. It was never like 'hey enter this room or plateau and fight everything in a giant circle until we let you leave'... it was more like 'uh oh, there's this nasty axe armor at the top of the stairs what do I do?'. Dark Souls nails that feeling.