I know, there's a lot of Abyssea threads already. But I think this topic would be a big help to figuring this out more clearly. If you get any information on an NM weakness please go ahead and post in this thread.
Helpful info includes:
-what color was the weakness exclamation icon?
-what job was the player who first interacted with the NM?
-what action seemed to trigger the weakness?
-what effect did the weakness seem to have?
-what drops did you receive?
Enemy Weaknesses:
In case you haven't looked into Abyssea info, all or most NMs have different weaknesses that can be triggered by using some kind of action on them, like flash, or a weaponskill. When a weakness is triggered a double exclamation graphic, "!!", will appear above the NMs head.
http://i35.tinypic.com/wb8mr6.jpg
What We Know From Game Text:
Here's what the various Abyssea related NPCs have to say regarding weakness:
Spoiler: show
Spoiler: show
Spoiler: show
Effects:
The NPC seems to imply that hitting a weakness can cause Atma to drop.
"Strike at a foe's weakness, and it will drop different spoils, often more valuable than usual. Among them you may find atma."
It's vague enough that it could imply Atma or equipment drops as well. But it's hard to say, and the only way to find out would be to keep track I guess. The text also suggests (and players have observed) that while some weaknesses can impair it's movement, another can limit its skills, and some have said that a weakness has made some NMs even more powerful.
Types of Weaknesses:
There's different colored weakness graphics that can show up when weakness is triggered. It's not entirely clear if different colors mean anything. Different colors could signify different types of weaknesses. Or perhaps it could change color depending on how tolerant the NM has grown to repeated uses of its weakness. Has anyone found these graphics in the .dats to see exactly how many/what colors exist in all? I think we know there's at least red/green/blue/yellow.
Possible Limitations:
Another natural question to wonder is, what kind of scope can an NM weakness lie within? The worst case scenario would be that an NM can spawn with ANY random action as its weakness. That would give you the smallest likelihood of happening upon a weakness and make it very influenced by luck. Iirc, we do know that weaknesses are tier specific. Meaning something like Fire II could trigger a weakness, where Fire I and Fire III wouldn't. (pretty sure that was confirmed correct me if I'm wrong)
It's worth considering whether an NMs weakness may be able to be influenced by any of these kinds of things:
-Job of the player that first interacts with the NM.
-Jobs of only players within the party/alliance that is fighting the NM.
-Weakness type of the same light that your party last gained before claiming NM (probably not, or at least Square didn't mention this in their lights Q&A)
-Each specific NM may have its own limited pool of weaknesses.