If a first-timer asked if you could give them all the tips and advice you could for Disney World what would you say?
e.g. which rides you definately should fastpass, how to get around the parks without waiting for hours in line, where to go to eat, what to wear, where you should go first. Pretty much anything, be their DISNEY GUIDE 
Can't wait to hear from you! 
Be at rope drop, run for the first couple hours to ride the best rides before the crowd hits.
My first advise would be to stay in one of Disney's resorts (if able), to take advantage of early entry into selected parks (again, if staying on property). Use Disney transportation, Dress comfortably, & research each Park, so that they can spend the bulk of their stay at the Park that fancy's their taste.
Same goes for dinning, there are so many restaurants, I am sure one will become their favorite! And, don't forget to make early dinning reservations!!!! btw, I love to eat at Boma, the food is exotic & delicious.
Pick the rides they really want to experience first, if there is any significant lines, get Fast Pass & move on to the ones with little or no lines.
The fun part of a Disney vacation is the planning, after they get to WDW, the magic & adventure begins. Cheers
Here's which rides I would say get a FP for immediately wen you enter for each park:
Magic Kingdom- Space Mountain or Peter Pan
Epcot- Soarin'
Hollywood Studios- Toy Story Midway Mania
Animal Kingdom- Expedition Everest
The other tip (besides arriving early/when the park opens, as mikesteffen previously mentioned... because that definitely helps a LOT) would be that you can grab another FP the second the time of your current FP begins before even using your current FP. For instance, if you get a FP to Toy Story for 9:50-10-50AM, at 9:50AM you can grab ANOTHER FP to Toy Story or any other ride before going to use that Toy Story FP. A lot of people don't know that you can do that!
Great tips, my biggest addition would be picking your time of year. If possible go during one of the lower traffic weeks. You can see all the amazing things Disney has to offer all year long with little competition for rides after a little home work.
My biggest tips for Disney would be:
-To save money, drive if you can (unless it would be cheaper to fly). The off-Disney property hotels are way cheaper, sometimes nicer (a regular La Quinta Inn is nicer than some of the rooms at an All Star Disney hotel (although I haven't been to one in a while because they tend to be so expensive).
-For Fastpasses, I always try to get ones the minute I arrive at Magic Kingdom for Splash Mountain and Space Mountain, which I've found to be the consistently longest lines in the park. The line for Space Mountain gets wayyy shorter if you wait until the end of the night (like an hour before closing), but even then it can sometimes be up to 45 minutes without Fastpasses.
-As Magic Days said, time of year is crucial. January is actually a great time to go, towards the end, but obviously not on New Year's day. February until President's Day is a great time, as is March until most public schools go on Spring Break. April and May are actually pretty empty, as most kids are just coming back from break and are about to have summer vacation. Going in the summer isn't awful, especially if you're not going to hit every ride. It just depends on how lucky you are with lines! After Labor Day until Halloween is a great time to go, as is November before and after Thanksgiving and December until most schools have break. You pretty much can tell when "off season" is by when Florida residents are allowed to go at a discounted rate.
-Look for hidden Mickeys!! It's one of the funnest things to do at Disney and they're EVERYWHERE! You can even make it a game and take bets with your friends.
My biggest tips for Disney would be:-To save money, drive if you can (unless it would be cheaper to fly). The off-Disney property hotels are way cheaper, sometimes nicer (a regular La Quinta Inn is nicer than some of the rooms at an All Star Disney hotel (although I haven't been to one in a while because they tend to be so expensive).
-For Fastpasses, I always try to get ones the minute I arrive at Magic Kingdom for Splash Mountain and Space Mountain, which I've found to be the consistently longest lines in the park. The line for Space Mountain gets wayyy shorter if you wait until the end of the night (like an hour before closing), but even then it can sometimes be up to 45 minutes without Fastpasses.
-As Magic Days said, time of year is crucial. January is actually a great time to go, towards the end, but obviously not on New Year's day. February until President's Day is a great time, as is March until most public schools go on Spring Break. April and May are actually pretty empty, as most kids are just coming back from break and are about to have summer vacation. Going in the summer isn't awful, especially if you're not going to hit every ride. It just depends on how lucky you are with lines! After Labor Day until Halloween is a great time to go, as is November before and after Thanksgiving and December until most schools have break. You pretty much can tell when "off season" is by when Florida residents are allowed to go at a discounted rate.
-Look for hidden Mickeys!! It's one of the funnest things to do at Disney and they're EVERYWHERE! You can even make it a game and take bets with your friends.
I think the BEST time to go, if you can swing it, which is difficult for people with kids who just went back to school, is the week after Labor Day. I went during that time a few years ago, by accident, I was just down there for work and decided to sneak in some WDW time, and I had the parks to myself. I could pretty much walk onto any ride I wanted and my hotel room (at a value) was unbelievable - I think it was like $52/night - which is a steal for Disney!
My biggest tip for Disney or any vacation is to be realistic. What I mean by be realistic is recognize & accept that you're not going to be able to see & do everything there is to do or see & more than likely everything you would like to do & see also understand & accept that the chances are that something will go wrong it might be minor (such as a mild sunburn or not seeing a lot of animals on the safari) or it might be major (serious illness or an emergency back home that requires cutting your vacation short) but chances are something not so magical will happen. Finally have realistic expectations, as in realize & accept that the kids will fight, each member of your group will have had an hour or a day during the trip when they're tired/hot/hungry/thirsty etc. & are just plain unpleasant to be around, not everyone you encounter will be happy & in a magical mood etc.