I haven't found a good FAQ--or any info in general--on Hotline Miami's scoring system, but I've managed to get an A+ rating on all but two levels so far. If you're trying to earn an S Rank, here are some tips for getting A+ ratings:

EXECUTIONS

Executions are the best way to rack up points. An execution is performed when an enemy is down (but not out) and you finish him with the space bar. Depending on the situation, you might need a few extra clicks to finish the job. There are generally three types of executions:

1. Barehanded
2. Against a wall
3. With a melee weapon.

Weapons are by far the highest scoring form of execution. Some weapons score higher than others. This is tricky because it means you have to knock down an enemy with your fists (or by throwing a weapon) and then quickly pick up a weapon before he gets up so you can execute him. Once you've mastered the game's controls though, it's easy enough.

Executing an enemy with your bare hands, or when they'e up against a wall, gives you the same amount of points, I believe, but considerably less than with a weapon. Note: You cannot use a melee weapon to execute an enemy that's up against a wall.

BOLDNESS

I'm not entirely sure what the game interprets as "boldness", but I think I've figured it out. If you punch an enemy as he's coming at you, you score way more points than by sneaking up on them. Usually, the best thing to do is to get an enemy's attention and wait for them to rush you, then, with good timing, stand your ground, punch him, quickly pick up his weapon, and execute them with it. Do this whenever possible.

I believe this also counts when you take out guys that are shooting at you. If you're standing really close to a guy with a gun, he'll miss, and you can safely punch him, but this is way too risky to be reliable. It's worth keeping in mind though.

AVOID USING GUNS

Don't use guns. Sure, once you've figured out how to properly use guns in Hotline Miami, beating a level becomes pretty easy, but they give you the least amount of points. In the ideal situation, you'd take out every enemy in the room with a weapon execution, so don't use guns. Of course, there are situations where it's unavoidable--some guys can only be killed with guns--those situations should not affect your ranking

GO BAREHANDED

You should be able to go into most rooms barehanded, take everybody out, pick up their weapons, and execute them all. So once you've taken everyone out in a room, consider if you'll need that weapon in the next room. Chances are, you're probably better off dropping it. It's tough, but that's what'll get you an A+ ranking. There are, of course, exceptions...

THROW YOUR WEAPONS

It's sometimes impossible to rush or sneak up on enemies with guns. In those cases, throw your weapon at them, then quickly pick it up and execute them. Sometimes there is more than one enemy with a gun, and I've found that if I time it well, I can throw a weapon at one, and quickly punch the other one before he sees me coming. It's one of the trickier things to do.

LURE

There are some rooms you can't really walk into. Either because there are guys with guns, or there's just too many guys. Learn to lure people out by quickly getting into their field of vision and having them come after you. Once they leave the room to come after you, you can execute them. This works particularly well for rooms with glass walls.

COMBOS

Combos are a fairly obvious way to rank up points, and the game definitely likes to encourage you for it, but don't rely on them. I've found that you get more points for taking your time and executing three men, than you would if you quickly took them out with a weapon and got the points for the combo. However, if you can quickly execute everyone in a room and string it into a combo, then definitely go for that. This can be difficult, of course.

SPEED

Speed only seems important in the context of killing your enemies before they kill you, or linking executions into a combo. I've managed to A+ levels while getting "0" as my time bonus.

VARIETY?

I'm not sure how it works, but the game's tips say that variety is rewarded. So I guess try to switch up the weapons you use as much as possible.

MASKS?

I'm not sure if the mask you use plays a role. I've only used the Richard mask to get A+ rankings.

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That's about it. If you have any tips, please contribute. Feel free to ask me any questions if you need clarification. Playing for points has been the most enjoyable things about Hotline Miami (besides the soundtrack). It's turned it into a kind of puzzle game for me--and I love puzzle games! Scoring high means analysing the room for patterns and completing each room quickly and efficiently. It basically pushes all the right buttons for me. And, yeah, it's pretty challenging too!

The mask that extends the combo window time is handy.

All I focused on when I got A+ on all maps was to always get a complete combo on each floor, or as close to one as possible. 
Using the mask that lets you kill people in one regular punch made it much easier to run through each room taking everyone down in one smooth motion. Sure, I had to restart a lot, but when I finally got the perfect run I pretty much felt like a god among men. 
A good trick to use on some of the levels was to wait with the execution when they wouldn't die immediately from just one punch and use that as an emergency combo extender if I went through that room twice.  
Never not moving, and always be punching.

There's a few rooms in the game you can chain into one combo with guns but not without (and one of these rooms is ridiculous to do without guns I think).

If you absolutely can't get a high enough score, there is an exploit-ish thing you can do.

1. Kill all but 1 guy
2. Throw an empty gun at him
3. When/if he picks up the empty gun instead of whatever he was holding, pick up that other weapon and toss it aside (optional, this is to prevent him from picking it back up)
4. While the enemy is standing there trying to shoot you with an empty gun, run up and punch him in the face.
5. Pick up gun while he's down and throw it at him when he stands back up (also optional, but this is another step to ensure that he only ever grabs the harmless empty gun, rather than going for a knife or something)
6. Repeat steps 3-5 until you have whatever score you want.

It is cheap, I'll admit, but I did end up resorting to this tactic after like 5 hours of repeatedly getting A's and A-'s on Crackdown (and it was the only level I had left to A+).

But as for getting points legitimately, I highly recommend either using the black panther mask (run really fast) or the chameleon (enemies don't notice you until you're right up in their face). With either of those, the chameleon especially, you can get away with FAR more reckless and direct shit. So for instance, a guard with a gun standing at the end of a hallway would normally notice you as soon as you enter his line of sight, and have plenty of time to shoot you as you run down the hall. With the black panther or chameleon mask, there's a good chance you could sprint straight at him and stab him in the face and/or neck before he can react. Opens up a ton of possibilities.

@BBAlpert said:

If you absolutely can't get a high enough score, there is an exploit-ish thing you can do.

1. Kill all but 1 guy
2. Throw an empty gun at him
3. When/if he picks up the empty gun instead of whatever he was holding, pick up that other weapon and toss it aside (optional, this is to prevent him from picking it back up)
4. While the enemy is standing there trying to shoot you with an empty gun, run up and punch him in the face.
5. Pick up gun while he's down and throw it at him when he stands back up (also optional, but this is another step to ensure that he only ever grabs the harmless empty gun, rather than going for a knife or something)
6. Repeat steps 3-5 until you have whatever score you want.

It is cheap, I'll admit, but I did end up resorting to this tactic after like 5 hours of repeatedly getting A's and A-'s on Crackdown (and it was the only level I had left to A+).

But as for getting points legitimately, I highly recommend either using the black panther mask (run really fast) or the chameleon (enemies don't notice you until you're right up in their face). With either of those, the chameleon especially, you can get away with FAR more reckless and direct shit. So for instance, a guard with a gun standing at the end of a hallway would normally notice you as soon as you enter his line of sight, and have plenty of time to shoot you as you run down the hall. With the black panther or chameleon mask, there's a good chance you could sprint straight at him and stab him in the face and/or neck before he can react. Opens up a ton of possibilities.

I'm glad I'm not the only person who had trouble on Crackdown. The rest of the levels seem easy to A+ in comparison.

@BBAlpert: Ah, yes! I forgot about that exploit. Yes, yes, don't be afraid to use this method. Though I've found the highest way to score points was to wait for a guy with a melee weapon to come at you, punch him, wait for for him to pick-up the weapon again, punch him again, and repeat.

And yeah, Crackdown was by far the hardest level to get an A+ on. Every other place was easy enough, but I couldn't do it until today. I even tried using different masks--especially the combo mask--but ended up doing it with the Richard mask and just learning to be more efficient. The difficulty I had was that the SWAT team would kill two or three guys before I had the chance to, which reduced the amount of points I could potentially score.

Thanks for all the tips. I'm sick of getting these C+'s.

Does this work for school too?

I love this thread, thank you for the tips!

I've been running through murdering everyone, racking up a full combo and still getting a B. Thanks for the tips; gonna have to change up my strategy.

I wonder if it's still the same in Hotline Miami 2.