I used to love it in the first 2 seasons, and perhaps even the third. I watched them from DVD in a few weeks or so during my exams, watching a few episodes per day. It was good because:
1- It had an element of mystery, especially about the island - you did not know how things worked or why things happened, and it did not trouble itself much explaining (Ruined in Season 4, which is all about explaining these stuff and creating more questions.. Too saturated)
2- There were a wide variety of characters whose intentions and which we were not sure of were revealed slow character development through flashbacks. (Ruined in Season 3 and 4, they became too trigger happy killing them and introducing new characters so quickly, as well as removing flashbacks for flashforwards.. which don't help much with char dev.)
3- It was science fiction enough to get us interested, but realistic enough to be believable (Ruined in Season 4, for obvious reasons)
Yet I'll still watch it because it has a few characteristics that are still good:
1- Very effective and good use of sound effects, for example using the screeching violins of Hitchcock to create suspense, sound effects in action scenes
2- Some scenes flow really well.
3- Some creative ideas
4- The dialogue can be quite good sometimes