For HI novices, the enchantment of going to any HI island for the first time is getting to know the culture, saying mahalo and aloha everywhere, going to a luau, etc. This can be done on any island, but I think Aulani does a great job in their offerings. It is also fantastic for those with kids (thank you Aunty's Beach House!). Pearl Harbor is something worth visiting at least once as well.
For veterans, Oahu gets a bad rap because of how busy Honolulu is and it is probably the least attractive island. With that said, I couldn't imagine only visiting one island over and over- they are all so different. The active volcano on the Big Island, the beauty, ziplining and other adventure activities at Kuaui (not to mention the most picturesque golf course at Poipu Bay!), the untouched splendor of Lanai, the over hyped beauty of Maui. I absolutely agree with Aulani being a "staging area" for other island trips.
Newbies should also be warned- there are a lot of run down areas on all of the islands so don't expect glitz and glamor everywhere you go