The best I can tell, Boldness and Combo are the categories with the highest scoring potential, closely followed by Killings and Mobility. Killings: Knock downs (whether with fists or thrown weapons) and executions are the bread and butter of a high score run. Not to say you can't take out the odd mook with an insta-kill to keep the combo going, but you'll want to try to knock down and execute as many as you can. Executions with melee weapons give the most points (locked at 1000pts) while unarmed gives less points (600, I believe). Some weapons give you more points than usual if you instakill (by throwing) or execute an enemy with them. These are the drill, beer bottle, hammer, ninja star, dart, and brick. The brick and the hammer are particularly useful because they can be thrown to kill a hostile (and anyone behind him) as well as used to execute enemies on the ground, both for a large point bonus (though the kill animation is longer than other weapons'). You also get more points for an execution if firearm enemies try to shoot you while you're executing one of their buddies (to the tune of 2200pts, I think), which I'm certain also boosts your Boldness, as well.
Boldness: The scoring mechanism for Boldness is pretty opaque. I think the game counts any time you are spotted and pursued by mooks to be an exposure. Being shot at also increases you're Boldness I'm certain. Taking on large numbers of enemies (that is, exposing yourself to a lot of enemies over a brief period of time) counts towards Boldness as well. It's hard to come up with any specific tips, other than to just be reckless.
Mobility: Another opaque scoring category. All I can tell you is that efficiency is the key. No wasted movement. This goes hand in hand with getting a high Combo score. There's not much I can tell you. You just have to experiment with a level until you find the fastest way through that chains as many kills together as possible, while still utilizing the techniques I outlined in Killings. Basically, if you can manage to combo entire floors, you can expect a high Mobility score.
Combo: This category is more transparent, as the score for each combo will flash on the screen. I'm not sure if you're given bonuses based on the size of your combos at the end of the mission or not, however. The only tactical advice I have is to eschew weapons with long kill animations and avoid getting pinned down by gunfire during an execution if you want to chain kills together. As I said before, efficiency is key for high combos. And again, you'll just have to experiment until you find a winning strategy for each level.
I'm doing a high score run of the game right now. Here's a link if you'd rather watch someone get a high score rather than read about how to get one.