How would you react if your husband-to-be said his ex-girlfriend's name at the alter? Answering this question honestly should help you determine how much you really hate Emily Waltham. Because that scene, where Ross says the immortal words "I take thee Rachel..." seems to be the point where Emily Waltham became a one-note caricature of the overly demanding wife.
Now, to be certain, just about any character who got in the way of Ross and Rachel being together drew ire from the Friends fanbase, but none of them irked people quite like the cute girl from Shropshire. Not even Mona, who we should all agree here and now was far more irritating and had a much worse accent.
People hate Emily because she freaked out on Ross for still being in love with his stupidly hot ex-girlfriend. Was she perhaps a little over-the-top in her demands that Ross completely cut Rachel out of his life forever? Perhaps. But she's allowed to be excessive because HE SAID THE WRONG NAME AT THE ALTER, YOU FOOLS.