Well I like to remind myself that Carmack isn't really that much of a gamer. He's a technologist. If he doesn't like a piece of hardware, there's usually something to it. I'd say it usually means that the hardware is poorly optimized for its intended task.
I highly, highly doubt he's partial to a particular vendor. The guy just likes things that make sense and are well-targeted to do what he wants. He has shown many times in the past to prefer open standards and doesn't exactly bow to these companys' demands. Heck, he developed Quake on a NeXTstation and preferred NT and OpenGL to 9x and Direct3D forever basically. It really is too bad D3D has won, but I think it was rather inevitable. It is entirely game developers' faults tho. He ported Doom to Jaguar and let people know the machine sucked. The GPUs we use today are at least partially based upon how his game engines have worked along with input for future 3D tech plans. Especially from NV, with all their optimizations for z-rendering (Doom3).
And, as has been said, Quake 3 was one of the very first games to ever make a stab at SMP support. He looked at the possibility, gave a shot at implementing it, and found that it wasn't going to be remotely simple or all that beneficial. And we've heard a bunch of other devs say the same thing rather recently.
(for illustrative purposes)
