re: What made "Lost" so captivating?

Beside some reasons already mentioned, the characters were unique and diverse basically covering every personality type.

This allowed for any given person to heavily invest in at least one character. Once they were invested two things happened:

1. Their stories intertwined leading the viewer to become more invested
2. The characters' back stories led the viewer to see other characters were really different than initially supposed...often times shifting those characters into the very type the viewer identifies. This made the viewer invest in more characters.

The story was just the vehicle for all this to happen. Think about what the finale REALLY focused on....character conclusions and relationships. That's what Lost was all about.

Alone we are all Lost, until we find relationships and really discover ourselves.