Unsummon was not always a universal ability, and in early versions of the game only the Necromancer had this skill. (It was inherent; not on his skill tree.) It was added to all characters are much complaining from Amazons who sometimes wanted to get rid of their Valkyries, and would have been necessary in D2X anyway, since the Druid and Assassin have minion skills, and various uniques and runewords were added that granted minion-summoning skills to all characters.
Unsummon is only active when your character has a summoned minion of some kind.
A minion hovered on with the Unsummon skill ready will show their hit points. This is the only way to view the hit points of individual skeletons or Revives.
Unsummon can not be used on a hired mercenary.
Unsummon only works on a character's own minions, and can not target or be used on the minions of another character.