New Monsters and Heroes: First off, HOTS introduces the Mummy, Djinn, Efreet and Ghoul monsters to play as, and the Djinn Trapper, Assassin, Gypsy and Maharib Town Heroes to combat them!
New Hunters: HOTS introduces several new mechanics, starting with new Hunters; the Yunfakh! They are fast and agile combatants, and cannot be attacked at range! You need to get up close to defeat them!
Also new tiles and new mechanics in form of Summoning are part of this Expansion.