Been to Disney World a few times.
My breakdown:
Magic Kingdom: Mostly boring. You can zoom through this in half a day if the crowds aren't bad. Space Mountain, IMO, is pretty boring. I like Big Thunder Mountain and Splash mountain much better. Do all the "theater" shows, like Fantasia - anywhere where they give you 3D glasses, is a must. Not only do they have 3d images but they have scents, wind, water, etc. Pretty neat.
Epcot: Mission: Space literally made me barf but it was fun. Soarin' is an excellent ride narrated by David Puddy off Seinfeld. One of my favorite things in Epcot is this igloo looking thing called the Coca Cola refreshment station or something like that - it has different coca cola products from around the world. Also, Epcot includes all the different shops and restaurants from different countries. My favorite is Morrocco but they have Italy, Japan, France, England, America, Mexico (maybe others I can't remember?). Epcot is probably my fave.
Animal Kingdom: The yeti rollercoaster (forget its name) was fun. A bugs life 3d adventure. The safari...etc.
MGM: Tower of terror, rockin rollercoaster, the stunt show thing. Star Tours is a waste of time IMO, check out indiana jones instead.
Always stay for the fireworks, every evening.
Get a fast pass for popular rides, you go to the ride you want, and get a ticket for it that tells you to come back at a certain time. It really maximizes the time you get to spend on rides rather than waiting in line.
Eat on the park - the food is surprisingly good, especially in the tour of nations in Epcot.
WHEN are you going? That will determine how bad the lines are.