if you have
{ a : [

{ x : 1 }

,

{ x : 1 , b : 2 }

] }
and do this update:
{ $pull : { a :

{ x : 1 }

} }
previously it would have only removed the first entry, now it will do both

you can also do things like:
{ $pull : { a : { x :

{ $gt : 5 }

} } }