The Adwaita theme and general ugliness of GNOME 3 is what put me off from using it for such a long time. Like, I was around when GNOME 3 first dropped, and gods was it ugly. I was very glad to hear the Linux Mint team announce MATE.
Yeah, sure, it was also a different design paradigm, but that didn't stop me from liking Windows 8 (which I bought when it first released, after having used all the previews). It was mostly how ugly it was.