Carmack, that guy who made some decent games back in the day and now is pretty much irrelevant because of not making anything interesting. The latest big release from id was what, Doom 3 BFG, a rehash? Ugh.
He doesn't like mantle but on the flip side, he said G-sync was good when one would have to invest in a new monitor just for that and it is also limited to PC. Mantle is open and so nVidia can implement it if they want to, just like AMD can implement something that works with G-sync. Ugh.