"I can't agree with Beyonce’s Irreplaceable as equally affecting as this tour de force, although the contrast is striking. Beyonce’s narrator as all-powerful diva who can dial up a replacement lover as easily as the cab she calls for the unfaithful one she's kicking out might conceal the hurt she might be feeling, but the melodic and thematic hook "You must not know about me", a kind of "Don't you know who I am" declaration of power and privilege suggests an over-indulged ego and doesn't inspire much sympathy or empathy as it aggressively asserts itself. "
See, I interpret "Irreplaceable" as being about self-delusion (Beyonce may not agree though!). The "I could have another you in a minute/matter fact he'll be hear in a minute") has this heart-tremor leap upwards which sounds totally poignant, like Beyonce's thinking "yeah, I could, but he wouldn't be you, and I don't know why but you're the one I've given power over me."
It's like there's this contradiction in the heart of the title: on a pragmatic level her boyfriend is totally replaceable (and to emphasise her point Beyonce sets herself up as the breadwinner in the song, as she would be IRL for any boyfriend other than Jay-Z), but there's also something about her cheating boyfriend which isn't replaceable, something above and beyond him as a person, and that's the him-as-object-of-desire (those of you playing at home can insert a reference to Lacan here). It's not so tangible or obvious that it prevents Beyonce from making the claim that he's replaceable, but it prevents the claim from sounding convincing to anyone, including herself.
This is why the song is so captivating: rather than be over-literal and grovel in her own anger or misery, Beyonce affects a pose of heartless indifference, and this affectation is much more affecting than the truth would be, partly because of the performative irony involved (we, the listener, know what Beyonce's boyfriend does not, which is that she doesn't totally mean what she's saying).
It's vaguely comparable to all those tragic stiff-upper-lip characters in books and films who have gracefully stepped aside, disavowing their unrequited love for one of the two main characters in order to make way for the central love affair. I can't think of any examples off-hand except for Kristen Scott-Thomas's character in "Four Weddings & a Funeral" - there's something tragic in the way that "putting on a brave face" is used to ruthlessly suppress one's own emotions.
