Notion: Modifiers suck

Modifiers are the essence of an RPG: some things are easier to do than others, and some people are generally better at certain tasks than others. Modifiers to the roll (or even without the roll, in a diceless game) represent innate skill or external circumstances that may aid or hinder your action. Even if modifiers are applied to a target DC rather than the roll itself, the Game Master still needs to track them.

However, modifiers tend to suck when there's a lot of them. They're easy to overlook: "Oh, wait, I forgot the +1 from the Bless spell! And I would have hit last turn if I'd remembered it; can we back up?" If you remember them, they take time to tally: "Let's see, that's a +4 from the Bull Strength and another +2 because I got Enlarged last turn; I'm raging right now, so that's +4 more..." And then there's issues of which apply when: "Oh, wait, I'm wearing a Belt of Giant Strength, so that means I don't get the +4 from the Bull Strength after all, since they don't stack." And then there's those modifiers that affect the calculations of the numbers already on your character sheet (rather than just adding or subtracting from them): "Okay, so that's a total of +6 to strength. That means +3 to melee attacks, +3 to damage, but only +1 to off-hand damage... oh, but considering my normal strength of 12, that's actually +2... now I can carry a heavier load and, since I was right on the edge, I'm no longer encumbered, which means I can move 10 feet further this round!" And all this d20 headache comes from only three spells, one magic item, and one class ability in effect!

Actually, I'm talking about more here than just a modifier: a + or - number. I'm talking more broadly about all effects--which can also include spell or magic effects, character abilities (including feats, maneuvers, powers, etc), and conditions (flanked, paralyzed, fatigued, etc). Even damage taken--especially to attributes/abilities--can be seen as an effect. Some effects (such as conditions or states) do not affect rolls but instead determine what actions are even possible.

So, with this broad definition in mind, the problem remains: how to streamline the tracking of all the active effects during a game? The first step is to look at what exactly comprises an effect:

Size (or condition)

First is there is usually some positive or negative modifier to a die roll. Alternatively, if a condition, the effect may instead limit or allow certain actions, or else represent a number of different modifiers. For example, being entangled might impose a -1 to all physical actions, reduce speed to half, and prevent all spellcasting.

Applicability

A modifier is usually applied to only a certain subset of rolls, such as only a certain skill or skill group. For example, a modifier might apply to all Jump checks, all saves, all combat rolls, or all STR-based skill checks. (As mentioned above, this applicability can be simple to add on to existing inherent character modifiers, or it may require recalculating other modifiers.)

Type (Stacking/Overlapping)

Modifiers are not always cumulative. Some may have a type or category that determines how they stack or overlap with similar modifiers. For example, in d20, natural armor stacks with mundane armor, resistance bonus do not stack (the highest applies), and damage or energy resistance overlaps.

Duration

Effects can either be inherent/permanent (determined at creation time from things such as skills, attributes, or even worn magic items) or circumstantial/temporary (determined at play time based on the current surroundings, spells in effect, relative positions of the characters, etc.) Temporary modifiers usually expire after a certain number of rounds, minutes, hours, or days. Or (even more of a headache) the modifier may vary over time, such as gradually degrade. (In general, it is temporary effects that require effort to track.)

Source

Effects usually have a source that determines how the effect can be canceled or manipulated. For example, magical spell effects can often be dispelled or nullified in anti-magic fields. Or certain defenses may exist if an ability damage effect comes from an undead creature but not if it comes from disease or poison. (Knowing a source often implies its type, applicability, and duration, but only indirectly.)

Visibility

Finally, there is the question of whether other characters/players can tell if an effect is currently in place. First, there's the question of how apparent effect is in the game world. Secondly, there's a question of how other players track global effects, even if those effects are not apparent to their characters. For example, one character may be exuding some 30-foot healing aura that other players must be made aware of whenever their characters enter the range.

Now that we've had a look at what goes into an effect/modifier, we can start thinking about how to simplify the tracking process.