My first Prius was the 2002 compact model. When I saw the larger 2004 model, I traded. I've driven it on long trips over mountains and averaged 51.7 mpg with a mixture of 70 mph Interstates and 45 to 55 mph 2-lane roads in the Mountains of KY, WV and VA. It has lots of room and is most comfortable and fun to drive.
Miles per gallon, especially when prices get past $2.50 per gallon. Also the excellent brakes. I had to teach my wife not to stomp a hole in the floor when I was approaching a stop light. She was used to having to brake much sooner than I have to with our Prius.
Lights that help see around corners when turning, especially on dark, rainy nights.