I've been to Kauai only once though I live on Oahu. I loved the beauty of the island and got to see most of the "points of interest." I'm not sure I will ever return, even though it's a 40 minute flight away (and not ridiculously expensive.) FWIW, I've never been able to stay on another island for more than a week or so (includes Big Island and Maui as well as Kauai.) I'm just too "citified" I guess.
I love exploring and seeing natural beauty - but once I've seen it - I've seen it. It's my (humble) opinion that (other than some very specific scenic or historic places) Oahu has just about everything one could want in an island paradise. I never get tired of Oahu and now I have many friends there. I know lots of beaches and trails which are virtually deserted - just like on the other islands. Most of the allure of Kauai (lush tropical foliage, beautiful beaches, hiking trails, mountains, etc.) are available on Oahu (though they are not as concentrated on Oahu.) I can live a lifestyle very similar to my old lifestyle (lots of things to do, concerts, movies, McDonalds - well forget that - trails, beaches, shopping, good (well, better) prices on everything, etc. etc. etc.)
The big downside to Oahu is lots of people (which mostly manifests itself as rush hour traffic.) If you can get past that, Oahu is a place I can live without getting Island fever. If I still lived on the mainland, I might once again make one of the other islands a "vacation destination." But now that I'm on Oahu, I just don't feel much need to visit those Islands I've seen. I DO want to see Molokai before I croak, but probably only for 3 or 4 days.
Every few weeks, I find something "new" on Oahu that I've never seen or done, so I don't feel particularly deprived in the exploration department. I will occasionaly simply drive up a street I've never been on before. Typically, I will see something of interest (a new ocean-veiw perspective, a trail, an unusual house, even an uncharted man-made rock formation - possibly used for worship or sacrifice.) I don't treat every day on Oahu as "vacation" but it still sort of feels that way.
Naturally, this is one man's opinion and I hope everyone DOES come see ALL of our islands. We need the tourist revenue!! (And you WILL enjoy the beauty.) We'll treat you very well no matter what island experience strikes your fancy. Ya'll come! As always, YMMV.