I'll be posting my opinions on Dark Souls here: what they did right, what they did wrong, how it held up when compared to Demon's Souls.
Gameplay[]
Dark Souls took Demon Soul's gameplay and polished it. I enjoyed the (in my opinion) superior weapon balance and greater variety of weapons and spells. As far as a hack and slash game goes, I consider Dark Souls one of the best.
I'm glad that they took out the world tendency and character tendencies in Demon's Souls. Those mechanics did nothing but to make it ridiculously time consuming and difficult to reach certain items, and made gameplay tedious for no reason. I'm also glad that they removed gender restrictions for armor, which was both sexist and pointless.
One complaint I have about DKS was the severe nerf bows recieved since Demon's Souls. Bows were hard enough to use competetively in Demon's Souls, but at least once you mastered it you could be a total badass, even at PvP. In Dark Souls bows are almost completely regulated to utility weapons. And certain things just didn't make sense. Why call a sniper crossbow a sniper crossbow if you can't even zoom in? What's the point?
Another thing I would have liked is some sort of option to allow you to replay boss fights and other segments of the game. I understand that they deliberately did not include saving, but I would have liked to revisit fights with bosses such as Sif, Gwyn, Quelaag and the like. In order to relive those fights, I have to either create a new character, which is a pain, or go to NG+, which can also be a pain. A built-in chat or a way to summon specific players, hostile or friendly, would have been nice too. I mean, they have this big lovely co-op system there anyway. I've heard arguments that they kept these things out because they wanted to make the game more difficult, but honestly summoning anyone makes the game a lot easier, so why bother holding back on petty technical nuances such as voice chat or party invites?
Atmosphere[]
Dark Souls opted for a much less scary and tense atmosphere than Demon's Souls. I both like and dislike this. On one hand, I feel much more comfortable play Dark Souls to blow off some steam. Demon's Souls is too dark for me, and too colorless and depressing. On the other hand, I do feel that my first playthrough of Demon's Souls was much more engrossing for me; people looking for a good suspense-filled successor in Dark Souls would likely be disappointed by its vibrant colors and sometimes cartoon-like enemies (giant rats? seriously?). If they ever make a new Souls game, I hope that they include areas as fun to hack and slash through like Anor Londo or the Undead Parish, but also include areas as terrifying and creepy as Demon's Souls 3-1 Latria.
The music was a big part, too. I liked the music in Demon's Souls very much: I had never heard anything like it. Astraea's theme just bled accusations and betrayal; the Tower Knight's theme had an amazing choir part, The Flamelurker's theme made me feel like I was in for a world of red, red pain, and the Fool's Idol fight had some of the most disturbing music I'd ever heard in a video game. Dark Souls just didn't really cut it for me. Most of them seemed to feel like the generic "epic boss time!" feeling. Notable exception: Gwyn's theme was absolutely amazing and completely unexpected.
Story[]
Both the games opted for a minimalistic story telling. I liked Dark Souls' story more. Demon's Souls had a clear cut good and bad ending, and you can pretty figure it out immediately. Dark Souls is far more ambiguous, and with every readthrough of the text and every conversation I have with NPCs and my friends, my views change again. It's a deeply enriching experience, far superior to anything I've seen in a long time.
Conclusion...so far...=[]
Being critical, here: I really enjoyed Dark Souls immensely. In terms of presentation, there is almost nothing finer in my gaming library. Gameplay is excellent and rewards skill but brutally punishes whiffs. But Dark Souls is not perfect; a couple of gameplay tweaks here and there, a better musical score, and more accessible (but not easier) gameplay would have all been improvements. Nevertheless, Dark Souls has brought the Souls games to the mainstream, in an honest and clean way. Here's to hoping for an even better successor in the future!