John Carmack is a coding genius - I'm not always a fan of the direction he takes - idtech5 stuff even organic models often has a chiselled in concrete look to them and that isn't just the art direction. But I've spent quite a lot of time beneath the hood of the idtech2 and 3 engines - when it comes to coding the guy is just on another level to most people - might be some better at specialised areas but his abilities to visualise, innovate and abstract, etc. are incredible.
I'm not sure hes had that much to do with idtech 6 latterly however - for awhile he was a bit burnt out I think on PC gaming development and looking for challenges elsewhere. I get the feeling to an extent what they are using in Doom 4 is more other people building on top of idtech5 and pulling in some more conventional techniques, etc. to update it as a lot of the earlier stuff that JC was talking about and would have needed some ground breaking work to achieve on todays hardware are entirely absent.