So, I have been playing DeS, nearly finishing a playthrough now. Impressions:
* Areas are much better done than DkS ones. I love the interconnecting areas of Dark Souls, but it seems that not having to worry about connecting them all made them focus more on level design or something. The areas are really enjoyable, large without being empty, and they look more verosimile. By this I mean that for example the castle looks like an actual castle, every room seems to have a purpose (thanks to amazing attention to detail) and the enemies fit right in (even more so than Dark Souls ones) which reinforces the feeling that the areas don't revolve around you.
Another thing I love is the lack of "always open" shortcuts which actually forces you to do the area and fight the enemies. In Dark Souls there are too many ways to skip content, such as the drop behind the Butcher in the Depths, taking the Silver Serpent Ring drop to skip most of the Tomb of the Giants, etc. I much prefer shortcuts that require you to complete the area to open, rather than simply knowing the trick.
A problem that some DeS areas have, however, is the lack of shortcuts. 4-2, for example. The area is fairly long, you're against nasty ambushing enemies, there are plenty of ways to fall to your death, and if you die, you need to do the whole thing again. I'm all for punishing players for dying, but this was too much. They should have at least let you unlock a shortcut to the boss. 5-2 is in the same boat, it does have a shortcut, but the area is insanely long, you get to the shortcut at the very end, and to open it you need to walk on an extremely narrow ledge while being attacked by annoying mosquitoes. If you fall there, you're at the beginning, so you need to do the whole thing again.
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*While DeS is the clear winner in level design, the lore of the game is much worse than Dark Souls's. The story is simple, it's narrated to you, and the game doesn't seem to give it that much importance. This is a shame because the atmosphere, as I said before, is incredible, it would be nice if the lore reinforced it. Dark Souls is worse atmosphere wise, but the setting is both detailed and subtle, even item locations often tell you more about the world. In DeS, however, item locations are mostly random, the lore doesn't seem ambitious enough to go past "serviceable", and there are a lot of facepalm-worthy mistakes, such as a contradiction in Ostrava's dialogue (he says Biorr was the one who fled Boletaria to warn the world, when it was Vallarfax) and the fact that Yurt is locked in a cage....but when you enter it, it becomes an elevator to proceed with the level.
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*The best for the last....difficulty. Everyone always seems to want to compare the two games, some people claim Dark Souls is harder, other people claim it's Demon's Souls.
DeS is both harder and easier than Dark Souls, in my opinion. It's generally harder, but the grass system and general unbalanced items and spells offset this increased difficulty and cause the game to be easier than Dark Souls. If you don't rely on them, however, expect a very hard time.
First, the game's lethality is through the roof. Plenty of enemies one shot you, some of them through your shield. For example, I did 4-2 quite early, found the Black Skeleton, and since it wielded katanas I thought I could block them. Serious mistake, he one-shot me through the shield, taking all my HP and all my stamina. This is only an example, but a lot of enemies are capable of doing insane damage. In this regard, the game feels less fair than Dark Souls, which also has difficult enemies, but less instant death moves.
Areas are more unforgiving, longer, traps deal more damage and most of the times they kill you outright, there are more enemies which ambush you and they attack in groups more often, then there's things like the fact you can't roll in the swamp, etc.
The lack of poise also contributes to making the game harder, you can be stunlocked, your attacks can be interrupted as if you had 0 poise, etc. The way the game handles the lack of poise is by adding a lot of hyper armor, which makes the combat a bit more trial and error, as you slowly figure out what moves grant hyper armor to enemies (which varies from enemy type to enemy type). In Dark Souls once you figure out how the poise system works, you're set.
Now, the cheese. It comes in too many forms to count, but from the Thief Ring onwards you'll have access to multiple ways of cheesing enemies. The Thief's Ring makes enemy aggro ranges ridiculously tiny, which allows you to pick them off one by one, you can run away and make them de-aggro easily, which drastically lowers the difficulty of the game, as DeS is all about hard levels and easy bosses.
To give another example, in my playthrough I was using the Crescent Axe, and I discovered that in this game knockdown doesn't grant the enemies any sort of immunity. This let me cheese enemies in absurd ways, for example with the Meat Cleaver Black Phantom I could pancake her over and over and over, and my stamina would regenerate faster than she could stand up.
The grass system is awful. It further decreases difficulty because it feels like playing Dark Souls with unlimited divine blessings. The worst that can happen is that you'll have to endure some tedium to farm the stupid grasses, but they heal a lot and you can use them stupidly fast. I guess this is the reason so many enemies one shot...if you do not get one shot, 99% of the times you can instantly munch a grass and go back to 100% HP. This makes the few challenging bosses the game has, like Flamelurker, really easy as long as you have enough grass, because you can keep munching it until the boss dies. Really boring.
Anyways I have more opinions on more specific things, but this point is already quite a long and disjointed mess as it is, so I will post them later.