I agree with basically everything MikeM said.
Like ChallengeMe, I never followed the series on TV (even this last season and the finale). I watched every episode in a condensed time period of a few months on Netflix and Hulu.
I'm glad it was open-ended and didn't answer everyone's nit-picky questions. I admit they caught me a little off guard when it was revealed by Christian Shepard that Jack (and everyone in the supposed parallel universe or "real world" of flash-sideways events) was dead.
I always had the hunch that the premise of the show had something to do with Hell, the afterlife, or purgatory, but I thought that place was the island. I think the writers tied most everything together nicely while leaving plenty of room for speculation and intelligent discussion.
I just watched it tonight so I still need to marinate on this for a while, haha.