It depends on what you're fighting I think, some opponents favor killing spellcasters and will even provoke (and some opponents have ways of avoiding provoking, like all the goblins have a way to disengage) whereas others will just hit whatever is closest to them, or what is lowest HP that they can attack that round. One thing that seems erratic is enemies attacking downed PCs, like instead of focusing the guy that's still up and going to kill them next turn they give a downed PC a failed death save?
And on the subject of death saves, IMO change this system somewhat. An ogre smashing into the ground gives you 1 failed death save. A phase spiderling nibbling you gives you 1 failed death save. These things are not equally threatening.
