I am still working over at my Game Chef 2009 blog on A Sea Deep, Cursed, and Rotting, putting together a rules summary for that game, which incidentally contains some portable rules for a character details module and an action effects module that I would use in conjunction with other games, including D&D. I touched upon a topic there that might be useful to get into in more detail here: effects layers.
Suppose you have a not-uncommon situation in D&D: a thief hiding in shadows, sneaking over to a place of concealment, then ducking when a guard hears a noise and throws a dagger towards the source of the sound. How do you handle attack and defense?
I'd tend to think of this as layers. The guard has to peer through the shadows to see what's happening near the crates, then examine the crates to note the thief's position, then aim the dagger at the dodging thief. I could have the thief player roll for the depth of the shadows at the time the thief hides, then compare the guard's Intelligence to the quality of the shadows; if the guard's stat is higher than the score for the shadows, the target gets a 1-point improvement to AC, but if the guard's Intelligence is lower, it's a totally random throw. The same could be done for the concealment aspect; I'd only have to roll Intelligence for the guard once, and only roll for the shadows and concealment once each, but I'd have to reroll if the conditions changed.
I'll have more to say about layers later.