Design Goal: Create a unified foundation and common basis on which to adjudicate special combat maneuvers.
I never liked 3rd edition's morass of rules for resolving combat actions like trips, disarms, sundering, etc. Whenever they came up in play, significant time was wasted looking up all of the pertinent rules and situational modifiers. Then, many cases resulted in either the player realizing he had little chance to succeed in his endeavor or that he had a trivial failure rate. So let's build a simple mechanism that these combat actions can all work from.
Whenever a character or NPC attempts a non-standard combat maneuver (such as a disarm or trip attack, anything that is not a standard hit to deal damage), the following mechanism can be utilized:
1) A normal attack roll is made. On a successful hit, the defender may choose to be affected by the special attack as described or they may elect to take standard damage instead.
For example, I declare that I will attempt to disarm my opponent. I make a standard attack roll and succeed in hitting. The defender chooses whether his weapon is thrown from his hand or he takes damage as a normal attack.
2) When attempting a combat maneuver, if the attacker's roll is equal to or greater than a 25, the defender is affected by the combat maneuver.
So a normal hit is going for Target 20, but if the attack hits a "Target 25," then the defender must accept the consequences of the combat maneuver.
This still requires (and leaves plenty of room for) DM interpretation. This mechanic covers the dice rolling aspect while allowing room for a DM and/or the players to come to a consensus on the results of special combat moves. With the dice rolling convention set, the following example could be used to interpret the results of a desired PC combat action.
Disarm. A success here throws the weapon from the defender's grasp. An unarmed attacker could wind up with the weapon in hand. If the attacker has only 1 hand free, he could end up with the defender's weapon if it is a one-handed weapon. Otherwise, the weapon falls to the ground. If the fight were occurring on top of a city's walls, the weapon could be thrown to the ground below. Or into a pit of lava. Or directly to an ally who is without a weapon. The defender should be able to retrieve the weapon if the situation permits.
That brings up a crucial point when making these sorts of rulings. If the defender can just pick up the weapon on his next action, the whole point of the disarm is made moot. The consequences for choosing to have the special affect should have a significant impact. As I said, this rule is meant to cover the mechanics of special attacks, but the real fun (and challenge) of D&D combat is coming together to form a consensus on the end results of actions.
Similarly, the defender's ability to choose whether or not to accept the special attack creates some interesting situations. In the disarm case, the defender may choose to have his weapon lost if he has a backup. If his backup weapon is not ideal for use in the current combat, then the disarm was useful from the attackers perspective. If the defender won't relinquish his grasp on his mighty +3 long sword, then he still took regular damage. The special combat maneuver will never be a moot point. When the attacker REALLY needs a success, he needs to be able to hit a Target 25, so he better be a good fighter or be able to leverage some other situation (beneficial spells, flanking, etc) to boost his chance at success.
Of course, situational modifiers still fit into this mechanic. If someone is attempting to run through an opponent, modifiers for size should be applicable when an ogre tries to bowl over a gnome. Heavy armor may hamper or help particular actions. At the end of the day, the whole mechanical side of these special moves in combat should be easier to figure out, allowing more time to be spent on thinking up these crazy schemes rather than doing book look-ups.
Edit: Ah, I found the original author of this idea over at Tales of the Rambling Bumblers.