I, at least, found myself tending to think of pit fiends as big bads that fight the party singlehandedly; others may have a similar mental stumbling block. Can you (or John) comment at all on how easy or difficult it will be to "upgrade" a monster to a higher category (say, from "elite" to "solo", for instance)? I imagine that higher "category"* monsters tend to get more powers to choose from (and more opportunities to use them).
*BTW, IIRC, monsters have a "role" (Brute, Solider, etc.); they also have the minion/regular/elite/solo categorization -- does that categorization have a name?