I don't mind that metallic dragons can be evil.
However, I
do mind that the ax doesn't swing both ways. A gold dragon can be evil but a red dragon, if one takes the
MM at face value, can't be good. An eladrin can be chaotic evil, but an orc can't be lawful good.
It's a double-standard really, and it bothers me. I'm all for moral grays. But when all it means is that the good guys aren't necessarily good while the bad guys are still evil, it becomes more of a back and dark gray setting than black, white, and gray one.
From a gameplay perspective, I understand it to a degree, as good parties are less likely to fight good monsters. Then again, I also dislike the "you should only play if you're a good or inclined-to-good guy" vibe the PHB takes as well.