This might be a ban-worthy statement, but...
It is possible to go to a park without planning anything in advance, strolling around doing rides as it seems convenient, not riding every coaster there, and still have a good day.
EDIT TO CLARIFY: Yes, I also mean that for first-time visits, possibly only-time-visits as well.
Also, I thought Thorpe Park looked very nice (lots of greenery I wasn't expecting!), Disney's Animal Kingdom felt too devoid of interesting things, and EPCOT was a pretty nice park overall.
Please allow me back in in time to make my ten-years-on-CF-post in 12 days or so...