let's see:
well first i want some good motivation not need to be an LOTR like story, but must be something viable, like if i'm a prisioner that will be tortured, then why me? what they want of it, there's something special on me or i'm just another poor soul?
second it need to make sense, i know that is fun to find potions here and there, but WHY they where on such place? maybe the torturer use it to make the victim live more time and prolong the pain? but it must make sense.
same for the loot, a bastard sword? dropping of an creature small than the sword itself? nope, it must be adequate, like gold found with people that probably don't even have pockets...
labirinths, they are labirinths? ok but why they are there, if it's an complex of rooms and chambers then it can't be an labirinth-like place, people that work there would go crazy.
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about characters, like the history, just some little basis to make sense how they can break throught the security system, and how they know to properly use some weapons and equips right of the bat, and so on.
I doubt that the EVILMASTER contracts 100% of fools to his ranks, so enemies also have to be some reasoning, i know some must be really dumb, but not all, let's say they are "street-smart".
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the gameplay sure must be team based, but hurts when each one one is 100% dependant of the others, it sure help the imersion, you're trying to scape the dungeon and must rely on your companions, but sometimes you will play with someone that picks up the healer, and will not heal the party! you will play with the guy that choose the warrior and will quit because he can't use lightning attacks like the mage, you will play with the stalker guy that don't use stealth and furtivity, in fact he will try to fight like an berserk driven barbarian. So gameplay must rewards team work and punishes solo adventurers that break the group BUT the group can't be punished along the "mr.iamtoogoodtocooperate, so someway to classes can supress the lack of someone is nice.
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and the worst, it must be fun to play, i usually play maps that i make and see if i can play for some good time, than i stop and think: i really liked it? i would play if someone else had create it? and if don't know whre is each single item and trap or how solve the puzzles the game would still fun? and the others can really pass the challenges without this "privileged information"?
i guess is that, hope it helps.
