The first two seasons of Lost are great because every single character in the sprawling cast was compelling, and, if not likable, then at least entertaining (the two weakest links were, coincidentally, the first to die). A major reason why fans felt a disconnect during the show's more maligned years is that all their favorite castaways kind of went sour.
Locke went from badass believer to gullible Island zealot and willing dupe, to the point where his eventual death by Ben's manipulative hand was inevitable. Kate, once belle of the isle, habitually ruined the best laid plans with a tired damsel in distress routine.
As for Jack? Someone exposed to bullshit like Ben's mind games, Locke's weirdness, Kate's flightiness, with daddy issues and bad nightmares of Bai Ling on a Thailand beach is bound to lash out every once and awhile. But for awhile the good doctor exceeded his rage limit on a weekly basis, and was to put it plainly, a dickhead, operating on a Yell First, Ask Questions Later basis that understandably got him nowhere all too often.