Lore and world :  Dark Souls >>>>>>> Dark Souls II > Demon's Souls

Gameplay : Dark Souls II > Dark Souls > Demon's Souls

Bosses : Dark Souls > Demon's Souls >>>>>>>>>>> Dark Souls II

PvP balance : Dark Souls II >>>>>> Demon's Souls >>>>>>>>>>>>>>>>> Dark Souls

I love how the world is connected in the first Dark Souls, the fast travel from the get go breaks immersion imo. the lore in Dark Souls was strong, the main bosses were introduced properly and were given great back stories, none of that can be found in Dark Souls II, it was just less of an adventure. Gameplay and PvP in Dark Souls II is better hands down, the stats and scaling are more organized. Movement like dodging is more precise, back stab is slower and harder to perform. Bosses in Dark Souls II were a big let down, no fight even came close to the epicness of Ornstein and Smough or Flamelurker boss fight.

Overall : Dark Souls > Dark Souls II > Demon's Souls

That's good to hear.  I'll admit that I was worried about stamina consumption while dual-wielding, so I'll tailor my build accordingly when I decide to try it out.

As for the positives and negatives of Dark Souls 2 compared to its predecessors, my personal opinion is that Demon's Souls has some of the best atmosphere (my personal favorite was the Tower of Latria), the first Dark Souls has the best lore, level design, and plot control, and Dark Souls 2 has the most refined gameplay and character building mechanics overall (although I sort of think that magic is a bit too strong currently in the game).

As for difficulty, I think that the original Dark Souls (particularly pre-expansion) is slightly more difficult on a regular, new game, while Dark Souls 2 is much more difficult on new game plus or joining the Company of Champions covenant (or even better, a combination of the two).  The Pilgrims of the Dark locations are also pretty tough on a regular new game, and Darklurker on NG+ and beyond can be especially painful.

I think the difficulty of Dark Souls 2 is fine, From Soft needs to find the balance between retaining hardcore veterans and not pushing away potential new players, NG is easier, but NG+ is lot more difficult and players have the option to make game harder through champion covenant and bonfire ascetics.

The biggest let down is the boss fights, there are a few good ones but I think many bosses are just poorly designed compares to the first two games.

Sorcery is okay, Hex is very powerful but the strongest Hexes require lots of attunment slots and consume souls when cast, so you won't make use of them until late game, which is kinda balanced I guess, Miracles are definitely too strong, lightning spears do massive damage, have a good number of uses, can be found in abundance (3 great lightning spear per play through), and the strongest chime is also the fastest, it's no surprising so many people are using faith builds.

If I had one complaint thus far is that bosses seem fairly easy.

It saddens my heart to say but this will be the last souls game I buy (and I bought all 3 early). There's something not right with it. I wasn't able to upgrade my weapons for 30+ hours in the middle of the game, and my progress was sheit. I don't even remember what I missed but there was something.
And now, I had to cheat to find out how to eliminate the poison from that boss in the mill, and I know that I would have never tried to do what it's required. It just never occurred to me that it's possible to do that (because it shouldn't be actually - not like that). I cleared that area and roamed those steps a lot looking for something...
I am disappointed. May this game design bring them lots of players, but they lost me.

I hate how us, offline players, have to farm Sunlight Medals which are something like 1% rate drop from Falconers in order to unlock the last miracle, if you're aiming for 100% -_-"

And why are you an "offliner" ? Co-op is alot of fun.

Desperate times in Spain? :p 

Desperate times + Xbox Live + Me myself not willing to pay for (and not having money, anyway xD) Xbox Live = No online experience in Dark Souls 2 for me!

I did play online the first week though. When Dark Souls 2 was finally out, MS gave a free Xbox Live Gold week for all users. Honestly, it was awesome. Don't know why they have online behind a paywall...

Coming from someone who never "cheated" in the first playthrough. I have to say that the game is hard and challenging, but not unfair. I didn't know about the mill too. Yet still, I managed to beat the boss in the poisonous environment after about 5 tries. Pyromancies helped. Whenever I fail, I try different strategy until one of them works, I almost never used Pyromancies before that boss. 

Damn, I can imagine myself in the same situation now Sony has walked this path with the PS4, agh. I will probably subscribe for it though.