This turned out to be way simpler than I expected. I could use more eyes on it though, especially testing it out in projects outside of the current mongoid test suite.

Referential validations no longer load the entire relation from the db with :validates => true being set.

If you want the old behavior, use validates_associated:

class Person
references_many :posts
validates_associated :posts
end

Since :validates => true is now a sane default, it's been turned back on in all the relations used in the specs.