Yes, the Fastpasses are your friend...making use of them wisely can mean the difference between waiting in miserable lines all day and walking onto most rides with a 5 minute wait.
I'd plan on spending a minimum of 2 full days in each of the 4 main parks if you want to see most things.
Don't miss the parades - all 4 parks have them, at least once per day - but you'll need to get a seat early if you are short or have kids with you. If you are tall you can get there late and still mostly enjoy it.
Also try and take in the fireworks shows in each of the parks at night - Epcot and Magic Kingdon and MGM all have one.
Stop and enjoy the live performances that popup as you make your way from ride to ride...don't rush by them just because you need to get on the next ride; they can be as much fun as the rides.
If you haven't' already, pick up Birnbaums Guide to WDW. Its worth it - read it and bring it with you; if you've never been there you can miss a lot and waste a lot of time if you don't have a plan.