Base building. Fortifying against enemies and being creative is a blast.
Exploration and big worlds. Games like Borderlands, Fallout and Far Cry with unique environments and ambiance.
Hate:
Escort missions. After all these years they’re still not fun.
Excessive health bars. Having to carry several different kinds of potions, etc. One of my favorite games is Dark Cloud for the PS2, but I think it had health, mana, weapon health, thirst and effects like poison that never cleared until you took a certain potion. I believe I used a GameShark or similar to get rid of thirst and weapon health.