Verno wrote on Jun 21, 2012, 09:10:

Completed nightmare yesterday. A more fun than normal but still a bit boring. Is it normal to not get a single rare from bosses the entire playthrough? Getting one from elite packs but not from bosses.

Yeah that was Jay Wilson's brilliant inspiration. He overruled the design team and said that there should only be guaranteed rares the first time you kill a boss on Normal which is why you see blues on the other difficulties. Nothing like killing Hell Belial and getting 4 blues. In a way it is almost indicative of their design approach in general with this game. They have this absurd obsession with getting people to run packs, as if pack repetition itself is somehow better than boss runs.