Okay.
To begin with, my expectations of the song were not high. I somehow knew it would be a poppier one, because it was announced to be a love song earlier. What's more, I'm not a huge fan of major key songs generally, and major key Muse songs are no exception.
Someone here says it has a powerful bassline - well, compared to killer basslines like that of Futurism it's meh. Someone says it's catchy - well, to me it's not. I've listened to Reapers countless times already, and I doubt the same will happen to me with Mercy.
On a positive side, the arpeggiated synths are very Bliss/Starlight/NSC-like, which is good. Wrapped in good production, Mercy can actually work. As of now, for me, it doesn't. But I didn't expect it to, so I'm not disappointed.