First thing that comes to my mind is audio design. It's something that's never really looked into and even in most of game reviews in professional magazines they tend to skip the matter entirely.
It would be cool to have some discussion on how to handle game audio well as its effective use can have huge impact on the game's immersion and atmosphere.
Another matter that seems important and usually not discussed enough is the game flow, linearity and exposition, both story and game mechanics-wise.