It's good for Miller because he'll get a fancy title, more money and presumably some added responsibilities, including expanded play calling.
It's bad for Miller because he's now officially accountable for an offense featuring no legitimate quarterback, the NFL's oldest offensive line and an elite receiver (Larry Fitzgerald) who is heading toward free agency following the 2011 season.
If the offense continues to struggle or fails to make anticipated improvements, coach Ken Whisenhunt will not be the only one taking the blame.
Therein lies an opportunity for Miller as well. The Cardinals almost cannot help but improve after suffering through an unusually poor season. If and when that happens, Miller's more prominent role will position him to reap the benefits. Things worked out pretty well for former coordinator Todd Haley in that regard.
Miller had served as passing game coordinator since Haley departed to become head coach of the Kansas City Chiefs. Offensive line coach Russ Grimm oversaw the running game. Having two coordinators was like having none, however, and Whisenhunt seemed to be the driving force behind the offense.
Miller becomes more of a focal point for issues relating to play calling specifically and the offense in general. He is officially accountable.