Well, I guess it's clear that opinions vary just a tiny tiny bit huh? DISCLAIMER: I've been diving in Hawaii for 14 years and am addicted so my opinions will reflect that just a touch.
I'm with Scottie and his feelings about reviews based on a handful of dives. At the same time I realize that you can only review based on what you know. I also can't agree more that each island has unique qualities and highlights. You're doing the right thing and reaching out for info which many visitors don't have time or the avenues to do. Knowing what a dive destination has to offer sets you up for exciting and rewarding experiences wherever you go. Scubaboard is awesome as a resource in this way.
While Cozumel vs. Hawaii may be like apples vs. carrots (oranges to overused) they are both fantastic destinations. Some of the complaints you here about destinations are not necessarily based on the location. What I mean by that is some complaints (valid as they may be) can be avoided with research. If you want unique locations there are often shops/DM's that cater to or at least consider those desires. If you want a small dive group - seek out a company that offers that. Private dives are often also available and not always cost prohibitive. Another thing that many divers don't realize is that you can request DM's that you have heard good things about online (SB, tripadvisor.com etc).
Sorry if I'm leaning off topic - my intent is to help with some angles that set you up for success.
Hawaii is such a unique location in that it is so isolated that as a result we have the highest endemic ratio of fish anywhere in the world. That is incredibly exciting for those seeking things they'll see nowhere else in the world! At the same time, being isolated means that comparatively speaking we are not a "fishy" location and massive schooling fish is not to be expected. The turtle diving here is world class! If you like diving with turtles - you just simply can't be disappointed with Hawaii. We have great inverts - and as a nudibranch junkie I love our little mollusks. Vis' can vary island to island for sure with the best consistent vis' being found on the Kona coast. That is not to say that the other islands are poor by any means. I feel comfortable saying that visitors can expect to be diving in the 50-80ft range commonly throughout the islands with certain dives/locations well over 100ft many days. Hawaii has great shore diving as well with Maui being a personal favorite. There are locations were you can dive with whitetip reef sharks, mantas, frogfish, see wrecks, deep vertical walls, lava tubes...... It really has something for everyone.
Check out marinelifephotography.com, seaslugsofhawaii.com, Hawaiisfishes.com
You can send me a personal message with any specific questions that you may have and I will be happy to help with anything you are curious about. I hope you'll find your way to the islands and see how fantastic the diving here is!!
Scottie the duck (Oahu) and ShakaDoug (Maui) seem very helpful too and I'm sure they would extend the same offer
Hope my little rant helps....have a fantastic day