I have been to Hawaii also, and used to be a travel agent, here is my take.
Maui - The best all around destination, especially if you are going for a shorter ( less than 2 wk ) time period. You can experience everything on this one island ( except the volcano ), but it is a great destination in itself.
Kauai - The most tropical of the islands, much quieter, less developed. ( The majority of Jurassic Park was filmed here ).
Hawaii - Getting better all the time. This used to be one of the least desirable islands to visit, not so anymore. Great beachs, natural beauty, and it's size lends itself to a variety of things to experience. The choice of resort is key here.
Oahu - Most touristy, overdevelopped by Hawaiian standards, almost urban. Some individual great spots ( Diamond Head, Waikiki Beach, Pearl Harbor Memorial ) though.
That is what I have learned ( in a nutshell ).
My favorite place in the US to travel, exotic, yet the people are the most friendly I have ever met, and the food is to die for.
Everything is pretty expensive, so be prepared, but this paradise is worth it, IMO!