I uh, really liked Jack for his philosophical debates with Locke and how the show unravelled him over the years. My favourite Lost episode is the Season 3 finale and it's great seeing him as this drug addicted loser post-Island.
I mostly hate him because he's just a rude person.
Some crazy stuff happens, his friends ask him what's going on and what does he say? "It's not important."
Every. Single. Time.
As if his companions weren't worth discussing important island mythology stuff with. It baffles me that they actually put up with that behavior for 5 seasons.
His obsession with wanting to fix everything on his own didn't help much either.