Having thought over the whole career of Lost, I can be satisfied with the outcome as long as some original timeliners survive, Juliet comes back (otherwise it wouldn't have "worked") and the following 11 mysteries are solved (and by solved I mean given an explanation, if not necessarily an all-encompassing one). Since "everything that rises must converge" I have not included asking what the connection between the timelines is.
1. MIB/Jacob - who are they, why are they doing what they're doing and how did they get their powers?
2. Ben/Widmore - why do they have a feud, what were their motives for everything they did and why can't they kill each other?
3. What is the origin and power of the Numbers?
4. How did Jughead blow the survivors from 1977 to 2007?
5. How did Jughead impact on Timeline X?
6. Who is the bloody-armed boy?
7. What is the sickness that has Sayid (and maybe Claire)?
8. What is the deal with Walt's supposed powers?
9. What is the deal with Aaron's supposed significance?
10. Why/how is Desmond special?
11. Why/how can Miles and Hurley converse with the dead?