Minions are returned to a player’s hand in the order in which they were played.
If the resulting total number of cards in a player's hand is greater than 10, the minions over 10 will be destroyed. Any minion destroyed in this manner WILL have their Deathrattles trigger. Blizzard has clarified that this is working as intended.
Transformed minions (Hex, Polymorph, Faceless Manipulator) return to the hand in their new form (Frog, Sheep)
Mind Controlled minions via Shadow Madness, Mind Control or Mind Control Tech return to the controlling player's hand.
Minions that are generated through other means are also all returned to the owner's hand, including the Shaman totems, Paladin's Hero Power (Silver Hand Recruit), Imp Master's imps, Onyxia's Whelps, etc.
Deathrattle effects do not occur (unless the player's hand is full).
Any buffs/debuffs on the minions returned to hand are lost.