Each island has its own personality. They are ALL gorgeous. We always go for about 10 nights as it's such a big trip.
Kauai is our least favorite. It's beautiful but too quiet for us. Too honeymooner-ish. We always spend just a couple nights on Oahu. Honolulu is a city with a famous beach (Waikiki). If you're going all that way, esp. for your first time, you really need at least 3 n on Oahu and you should go there to say you did, at least once. Pearl Harbor is both stunningly beautiful and one of the most moving experiences you could ever have. The last time we went we also got to tour the USS Missouri which was really neat. Pearl Harbor can occupy a good bit of a day if you go early and stay for the Missouri. Once you have "been there / done that" you can decide if you ever want to return to Oahu. I agree that the new
DVC resort at Ko'Olina will be QUITE different since it won't be right in Waikiki - it will have more of a resort feel.
The Big Island of Hawaii is very different from the others. It has the active volcano, Kiluea, and offers a fantastic opportunity for sightseeing with helicopter rides that include flying over Kiluea and seeing some beautiful coastline and waterfalls. REALLY, really pretty. It also has different climates. You can go from rainforest to desert in the blink of an eye. It also has the Black Sand Beach and some FANTASTIC snorkeling right off the shore (at Keauhou). We're happy with about 3 nights on The Big Island.
Maui is our absolute favorite island and we always finish our trips with about 4 nights there. We have stayed in Kaanapali and in Wailea. I think that the resorts in Wailea are considered more "upscale" but we were perfectly happy with the Sheraton at Black Rock in Kaanapali the last time we went. Beautiful beach, nice property and just a 10 minute walk to Lahina. Maui has a lot to offer with sightseeing (The Road to Hana, Haleakala Crater etc.) and it is also great for just relaxing and enjoying being in paradise.
If you golf I'd suggest trying to play a round on Maui somewhere (you can always arrange to rent clubs). I surprised my DH with a round for his birthday one time. Before we went I called the concierge desk where we were staying and the girl hooked me up with where to play and with a guy to rent clubs from. He was REALLY surprised.
It's a tough decision for your first trip on where to go. There is a lot to see and do so you really need to go for as long as possible so you can squeeze in some time for relaxing too! If you can go for 2 weeks that would be your best bet!

Too bad DVC is switching to RCI - you just lost some great trade possibilities on Maui that were only thru II. Aloha.