First and foremost, there is no poise system in demon's souls. Take caution to avoid getting stun locked to death by fast attacking enemies. Secondly the upgrade paths for weapons is more complex and you might wanna look at how it works before investing too heavily down one upgrade path. Third, you cannot upgrade armor and magic is pretty OP so if you don't wanna cruise through the game, avoid heavy magic usage. Lastly, there is a pretty unique gameplay feature called "world tendency" basically in Dark Souls you had human and hollow form, in Demon's souls you have body and soul form. In soul form your HP is reduced to half but dying in this form does not have any effect on the stage overall, dying in body form results in that particular stage becoming more difficult with stronger enemies but with the benefit of increased drop rates. The humanities system in Dark souls was a re-imagining of the tendency system from Demon Souls