"We need to talk" will often be said after toilet seats are left in the upright position, and/or you come before she does and/or in the event of forgotten anniversaries.
I need to dump you, and it's suddenly "our" problem/fault. Often said to be a girl phrase, is in fact used just as often by guys. When employed in e-mail, is an attempt to simultaneously sever e-mail contact and preclude actual "talk"...
emails: We need to talk. I'll call you.
Never quite summons up courage to call. Hopes like hell she won't e-mail or ring, and that he can skulk off into the night. Sign of cowardice, therefore an automatic disqualifier when it comes to relationship material...
She means "I want to whine, bitch, and nag about something you did, known and unknown to you!" It can also mean that she has nothing worth saying at all. Be prepared for a half-hour impromptu sermon on whatever she has on her mind (and that would me many random things).
It is especcially annoying when she uses the word "we", knowing damn well that SHE is the one who will be doing all the talking.
Susan: Honey, we need to talk about your insensitivity to my feelings.