Back when I was a part of the BYOND community (yearrrrs and years ago) one of the members created a game called Mystery Mansion. It centred around the idea of an online, multiplayer, top-down horror game in which every game had a series of rounds, and each round one player was picked to be the murderer. This player was informed at the beginning of the round that they were the killer, and it was their job to kill the other users without being seen. "Being seen" would occur if you were in the act of murdering a given player, and any other given player had both you and the player being murdered in visible view.
To add to the worry of wondering who was going to kill you, they incorporated a line-of-sight system that only allowed you to see a cone of tiles in front of you, meaning you were always checking your back.
The game was especially awesome because of all of the different ways someone could die. You could set up booby traps, push people into fires, hide in closets or behind plants, there were dozens of ways that the murderer could get you. Even better was the fact that just because the murderer might have been witnessed committing the murder, the game wasn't over unless the witness could make it to a phone somewhere in the 3 story mansion and call the police, meaning if the murderer saw you witness the act, you would have to high-tail it out of there before he murdered you as well. It was like an extreme game of Clue, and highly addictive.
So take from that what you will. Line-of-sight is very important in top-downs to give a sense of urgency, so I would start by developing a very good lighting system that only allows the user to see what they could normally see. Murder Mansion had a very intelligent lighting system; not only could you only see in front of you, but you couldn't see behind closed doors or tall objects.
One last thing I distinctively remember from the game was the ambiance of the sound effects. It was always midnight and raining outside, and in going outside you really got a sense of being out in the middle of a storm with a murderer on the loose. The attention to detail was amazing, if you walked in a puddle of blood, your character would leave a trail of bloody footprints for the next 30 or so steps he took, the same goes for coming in from the rain.
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Here's a video of the game in action:
http://www.youtube.com/watch?v=auOM6r2NnTk
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It appears the video is just of a person walking around in an empty game, oh well. Brings back memories

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