Act 1 , scene 1, Lear says in L81 " Nothing will come of nothing." How is this threat significant for 2 other characters in the play?Lear is...

I think that one does not have to go far in assessing the
statement with the likes of Regan and Goneril.  Both of them had set their sights on
"the old man's" kingdom for some time.  Their flattery, as alluded to in the subtext of
the question, is designed to develop a veneer to showcase their "love" for their
father.  In this light, they perceive "love" and the words associated with it as a means
to an end.  Both of them understand that this is the way to accomplish their end goal. 
When Lear speaks to Cordelia about "nothing will come of nothing," it is significant to
the older sisters because they fully understand two realizations.  The first is that
their plan is working and has to continue.  The second is that Cordelia can now be seen
as expendable as her designs have come of "nothing" in comparison to the older sisters. 
In the largest of senses, the threat and the entire scene creates a very intense picture
of what love is defined as and what it means.  Lear associates love with words that
flatter and believes that the ornate decoration that might befit a man of his title
represents "love," something that Regan and Goneril manipulate to their advantage.  At
the same time, we see that the love which speaks of "nothing" is actually the most pure
and true form of love, and within this, there is a significant threat that Lear issues
to Cordelia as a response, but also a threat is present to the older sisters.  Regan and
Goneril have to work quickly to dispense of Cordelia lest the father realize that the
one who spoke of "nothing" actually possessed more than those who professed to speak of
"something."   When the idea of "nothing will come of nothing" emerges, it is quite
telling as "nothing" comes out of Regan and Goneril's affections and the only
"something" to emerge is from the one who initially spoke of
"nothing."