I'm addicted, I'll admit it - I started watching LOST online because all the past seasons are posted on ABC.com and now I'm hooked!
The thing that really pulled me in was the characters. Since the whole show takes place on an island, the setting isn't really much of a draw, but the characters totally make up for the lack. You are instantly connected with them and their previous lives which make you emotionally involved in their 'current' lives on the island. I especially like how their lives kind of intertwined before the crash but they never noticed. And now that they're all stuck on an island together they have to befriend people they probably wouldn't have ever talked to back in the 'real world.' I won't give too much of it away, but the dangers they face on the island bring them all closer together. Go check it out. I'm a fan.