Lets take a look at what Demon's Souls offered us:
Some of the best, most varied level designs of the generation. Take a look at some videos and pictures - each world had a vastly different art style with varying themes and styles, all beautifully crafted with some gorgeous enemies and bosses. But not only is it good looking, it's effective - everything is clear and concise. You can tell at a glance what you are going up against, and can easily take in the entire level - I cannot think of a situation where something happened that was not obvious.
Probably the most "honest" difficulty of the generation. Where most games manufacture difficulty by upping the numbers and giving the enemies a significant advantage, Demon's Souls provides the exact same chances to you and the enemies. They hit hard, you hit them hard. If you die, it's because you made a mistake, and with the exception of one area of one level, there is no point where an enemy holds a distinct advantage over you - It's all in how you play the game.
So then we get to how hard it actually is. Because of this system, every victory feels rewarding, even against some of the smaller enemies. There's a sense of accomplishment to every level, because you work for it, and when it goes well you know why.
TL;DR: Buy Dark Souls because Demon's Souls was an incredibly unique and well made game and Dark Souls will be the same.
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Joyrock
Fresh from my first justified ban. Ever!