For me I think it's the idea that more can be possible in sci-fi & fantasy than in the mundane world. Explore a floating castle in the sky, fight with high-tech gizmos, ride a dragon, or whatever else. 

At this stage in my lit journey, I prize good writing, which can be difficult to find. I'm currently slogging thru Wheel of Time, which has an interesting plot, but the editor needed to take a stronger hand in helping Jordan with his style. An exemplary set is Rothfuss' Kingkiller Chronicles, but, alas, it STILL isn't done. 

One thing I identify as good writing is a character who can make you laugh or cry. A book that can create emotion is a win for me. When Samwise says, "I can't carry the ring for you, but I can carry you," that's a line that stays with a generation of readers. Eragon's unrequited love of Arya seemed well done to me. Name of the Wind is absolutely chock full of emtional encounters with the main character as he makes friends and enemies. 

 Kingkiller is great. Decent chance we all die before book three comes out. :/