McDermott is clearly a good coach. I think the frustration with him is that he could be a great coach. Even more frustrating is that the things he needs to improve to become a great coach are easy and obvious. He has the hard parts of being a great coach down: motivating and teaching players, xs and os, hiring competent people to surround himself with.
Some of his in-game decisions are so bad it's impossible to defend. Wasting timeouts left and right including the one where he called a timeout to decide if he should challenge. All the 4th downs in the Tampa game were botched. Punting late in OT in the snow game when a tie would have knocked us out of the playoffs, called a timeout erasing Josh's 4th and 1 conversion today, etc etc. All of these things are incredibly stupid, but can be corrected. It reminds me of the term 'you can't teach speed' You can teach an otherwise good coach that it's smart to go for certain 4th downs but it's much harder to make someone who already knows that into a great coach. It seems like he has to be forced to be aggressive, last year before the AFCCG he seemed to have improved, but regressed in that game and has carried over to this season with the exception of a few games.
Another frustrating aspect is that he seems to be more aggressive against bad teams, and more conservative against good teams. Most fans think it should be the other way around especially if the other teams strength is offense.