I'm bored at work, so I thought I would list all the unsolved mysteries (according to Lostpedia)and whether they're important or why they won't be solved.
The Source- As lame as this may be, I think me might have to take "Mother's" explanation. Hurley's Bird- I haven't seen or heard any referance to this since it was 1st seen. It's not important. Cabin's Resident- All signs point to the MIB living in the cabin. Supply Drop- The Dharma story is over. It's not important. Glass Eye- Ditto Walt's Powers- Again, Walt's story is over. As important as it might be to some fans, I doubt it's important to the producers. Outrigger chase- No idea what this was but I don't see how it will be addressed. Passenger Massacre- All sings point to Widemore. Sayid's Assailants- I hnoestly don't see how this is important.
Thats pretty much it. The only ones I see being tied up are the flash-sideways timeline, of which only the Island submergence I feel is semi-important.