I've been watching Season 1 on DVD again for the first time in years and I had forgotten just how wonderful that first season really was. The chemistry between the Core Four (and especially Alex O'Loughlin and Scott Caan) was there from episode 1, and there was a tremendous forward momentum that kept everything moving along to the spectacular season finale.
Hawaii Five-0 is a textbook example of "fixing" a show that wasn't broken. In Season 2, we went from the classic chemistry/team of Steve/Danny/Chin/Kono to the non-chemistry/team of Steve/Joe/Lori/Max. I remember at the time thinking, "What's this new show? What happened to the old show that I loved??" The root of Hawaii Five-0's decline began right there because a lot of people gave up on the show when the new characters (especially the Lori character) were shoved down our throats.