I'm not saying that there aren't unanswered questions. I just think there are far fewer unanswered questions than a lot of people seem to be implying.
Well I deliberately kept a notebook of every running question I had over the course of the series--I knew that with so much minutae and years passing I would never be able to remember everything across 6 seasons given the highly complicated and densely plotted storytelling--LOST wasn't a show with one or two big mysteries you could keep track of.
This spring I went back through everyone of them and yes many were answered--some were a step in answering part of the original question raised, others were more complete-but there were still several unanswered. I have mentioned them several times in several places and I don't want to do that again here but needless to say the Mythology feels only partially finished and therefore incomplete.
I know it is hard for some fans to accept--and you don't have to agree with me--but Mythology, contrary to popular belief, was, unlike BSG for instance, a very important part of what made LOST--LOST. So much so that tv series like Heroes in season one or Flash Forward copied it. Heroes became a mess ultimately, however, I can't get too mad at it since the damage to that series was kept away from season one's brilliance. Heroes' writers wisely set up each season as its own self-contained volume with its own self-contained questions and gave us those answers by the end. There were one or two mysteries left dangling--who is Jessica? What was Kaito's ability?.
LOST, on the otherhand, did treat each season to its own volume in a sense with its own new recurring characters, mysteries, questions etc and did answer them however given the extremely interconnected nature of LOST those seasons were related to the others in a myriad of ways so when certain things aren't answered the damage isn't isolated to one area but spreads in the way it hurts some of the other pieces.
So I know the latest mantra is "it is all about the characters" but a lot of S3-5 was heavily PLOT/Mythology--tons of exposition, pulling threads together etc. Look at season 5-in terms of analysis it would mostly be a synopsis of what happened--there was character work but not a lot in my opinion. Lot of action, lot of teasers--Jacob/MIB, the outer wall of the Temple, lot of filling in gaps, lot of providing dates and letting the timeline of the island's history take shape, Eloise explaining the Lamp Post, mystery of Locke, characters reaction, being moved around, quickly reuniting, searching for answers, who knows who who knows what and when, feverishly advancing the plot, jumping from multiples in a given hour, lots of twists, cliffhangers, explanation of the rules of time travel etc
LoSt was a very complicated series so I shouldn't expect that my analysis is any less clear cut and complicated.