Just my own personal preferences, but what really makes another person's trip report enjoyable and useful for me are the following:
-Give the profiles of those traveling with you so I have a frame of reference for your opinions (i.e.-families with young children have a much different perspective than adults traveling alone)
-Provide information that would be relevant to others; skip that which would not be (i.e.-I do want to read about your restaurant experiences, but I don't care to read about the Pop-tarts you had for breakfast in your room)
-Provide descriptive detail but keep it brief (i.e.-I do want to read your impressions of the hotel room but I don't need a run down of every piece of furniture that was in it)
-Tell me WHY something was good/bad (I find reviews that merely state "it rocked" or "it sucked" to be useless; tell me how I can have the same good experience and how I can avoid the bad pitfalls).
-Share any helpful tips you figured out or tested along the way
-Share at least a synopsis of your experiences in all the main categories of concern to most WDW travelers: hotel, dining, transportation, attractions
Like I said, those are just my thoughts. Just share your experiences in whatever style you are comfortable with - I'm sure no matter what kind of review you do, everyone here will be very appreciative of your efforts!