Personally, I don't think Keen translates too well. D&D5 is designed to have a static 5% crit rate, and since there are abilities that can trigger on crits, that might make it too powerful. (Great Weapon Fighting feat) ... Now, Keen generally means "Sharp" ... so rather then doubling damage...
It took my group 15 rounds to get through the last of the 3 zombies. One of them nearly died in the process, and the Paladin did not have any spell slots remaining for Smite Evil, so the zombies wrecked the group. I wonder if the paladin's player learned the lesson of not eating all their spell...