I like creating interesting challenges, and then seeing how the players solve or overcome them. "How do we sneak past the guards in the middle of an anti-magic field? Our usual tricks won't work this time..."
I like world-building: creating a vibrant, detailed, and interesting world for the players to explore. "I wonder what lies beyond the Wailing Wastes? Nobody has ever returned from there...at least, never returned with their sanity intact..."
I like character-building: watching the player characters go from level-one Nobodys to high-level Heroes, one quest at a time. Hercules and Cuchulainn need not apply; we are looking for Samwises, Pippins, and Frodos.
I like creating my own monsters, to play to the party's unique strengths and weaknesses, and toy with the players' expectations. "I thought you said this ooze was immune to lightning damage!" "The last one was!" "But the last one was black! This one is canary yellow with green spots!!!"
I like writing my own magic items, to reward the players in more interesting ways than "a number goes up." A magic shield that cuts the random encounter chance in half...a magic tent that can cast rope trick once a week...a potion of healing that refills itself at dawn....a vial of endothermic fire--it's like alchemist fire, except it does cold damage...