When describing this game to customers at work, I tell them that this game is "a good burn". The challenge is wonderfully brutal and punishing. the game does not hold your hand at all and there is a real sense of danger in the game. The combat has a really nice heft and weight to it. It's also extremely exciting when something huge and terrifying comes after you.

The world is gorgeous
The monster designs can be disgusting and amazing in all the right ways
The combat is tight, responsive, tactical, and fun. 
The music is just downright terrifying in certain boss battles. 
Some of the areas (The sewers with all the frog monster things ) are really kind of discomforting both because of the atmosphere and because you have no idea what will come at you next.
 The bigger boss battles are exhilarating.   
Exploring the massive world is just plain fun.
 
The challenge and the difficulty aren't really what keep me playing the game ( The game really isn't that hard) .  I just keep playing it because I love absolutely everything about it.

I do really like the challenge but I also just really love the world, the combat and to be honest I love just finding all of the little hidden things in the game and the world .Finding and crafting those super good boss weapons and stuff and eventually using them to go invade other players worlds... I love it. 
 
Sadly I got in Demon Soul's a bit late and was never able to do a lot of PvP. I really hope that I will be able to do it a lot more in Dark souls and I hope that a good pvp community fosters out of this game. But yeah I just think it's a brilliantly done game and it's honestly the most addicting console game I've played in years. (dark souls). Like I've already put so many hours into this game, its crazy. 

Other games feel like progress is artificial.  In the JRPG I leveled up so I'm strong now.  In the WRPG I made the super armor and found the secret sword so I'm strong now.  In Demon's Souls it's just,  I learned a lot and I have the bruises to prove it.  Come at me bro.  Any time you slip up, the game will remind you really fast just why you were doing so well before.  Not because of your level, or your gear, but because you had your shit together.
 
It's an RPG that can give you the same high that a fighting game can.  We need more of those.
 
I can't love Dark Souls, yet.  It's still in the mail.

I liked Demons Souls because it reminded me of Megaman with choosing which areas to take on. Dark Souls is much different in that respect but I also enjoy the crafting, and unique characters in both games.

• I love the dark fantasy fiction
• I love how you are an ordinary warrior and every new discovery you make, be it a boss or an area, enhances the feeling of "I'm fucked"
• I love how you are on your own and have to be responsible for your every mistake
• I love the tense atmosphere and the feeling of how small and pathetic you are compared to everything around you
• OH GOD WHAT THE FUCK IS THAT
  

I've only played Dark Souls for 10 hours, but based on that:

The equipment, monster and sound designs are amazing. Those combined with the unforgiving nature of the game makes for a tense atmosphere, I was genuinely unnerved when I found myself in a place I probably have no business in for another 20 hours, full of monsters I don't even want to get close to.

If you keep your wits about you you can survive even without prior knowledge of what's ahead, don't push your luck and you won't get fucked. When I finally lost my first 11 humanity it was because I rushed, I deserved it.

I guess this is specific to Dark Souls, but the way every place (so far) is interconnected is great, the world feels natural and coherent. Feel like taking a hike to the blacksmith from wherever, go on ahead.

Last but not least - I can finally play the damn thing.

The atmosphere, the combat, and the whole risk/reward system.

I liked Demon's Souls because it was a good game, and I like those. 

I learned 3 things about Demon's/Dark Souls:

1.If you die it's your own fault

2.If you die it's your own fault

3.If you die it's your own fault

The atmosphere, the combat,exploration, loot, craziness

1) Its a game i can really immerse myself in. The setting is fantastic, its dark and keeps fantasy sensible.

2) Its refreshing to be walking around using a claymore and regular knights Armour opposed to DEATHKILL - AXE OF THE ALL EATING wearing neon shoulder pads. The combat is also extremely malleable and rewarding.

3) Its mysterious. Very few games feel like the exploring im doing is my own. Its a game that doesnt feel like im following a bread crumb between story elements but rather makes me feel like every discovery i make is my own and im the only one whos ever experienced it.

4) Its challenging and rewarding. The game is absolutely unafraid of making it hate you. It will punish you for small mistakes and make you a better person because of it. It doesnt take it easy on you simply because you are on low HP or because you dont understand the importance of block or parry. Each boss doesnt have a 10 second attack window with a giant red HIT ME sign, you have to experiment and find out what parts are vulnerable and at what time.

• Atmosphere
• Designs (armour, world, creatures, sound etc.)
• Lack of hand-holding
• It's Satisfying
• Punishes me for making mistakes
• The tension

1) Its a game i can really immerse myself in. The setting is fantastic, its dark and keeps fantasy sensible.

2) Its refreshing to be walking around using a claymore and regular knights Armour opposed to DEATHKILL - AXE OF THE ALL EATING wearing neon shoulder pads. The combat is also extremely malleable and rewarding.

3) Its mysterious. Very few games feel like the exploring im doing is my own. Its a game that doesnt feel like im following a bread crumb between story elements but rather makes me feel like every discovery i make is my own and im the only one whos ever experienced it.

4) Its challenging and rewarding. The game is absolutely unafraid of making it hate you. It will punish you for small mistakes and make you a better person because of it. It doesnt take it easy on you simply because you are on low HP or because you dont understand the importance of block or parry. Each boss doesnt have a 10 second attack window with a giant red HIT ME sign, you have to experiment and find out what parts are vulnerable and at what time.

@ahaisthisourchance said:

I learned 3 things about Demon's/Dark Souls:

1.If you die it's your own fault

2.If you die it's your own fault

3.If you die it's your own fault

Hahaha, I die alot and I always say its my own fault. I dont think I have ever got angry at the game yet, just at myself for not noticing my stamina is low so I cant run away before something jumps on my head and kills me >.<

The Soul's games are difficult, Fun, great design, source of satisfaction when you do something hard, and a way to get away from the everyday style of gaming.

Brutally tough but fair.