[BigRedChief]

Since Charlie first asked "where are we?", Lost has been a show about mysteries. The narrative has been slowly developing as we discover more about the Island, its powers, and its inhabitants.

Now in its fifth season, and preparing for its last, the show has gone through high points and low points. We refer to these as "Desmond" and "Nikki," respectively. But with more questions being answered every week, and fewer new ones being asked, it's clear that the show is in its autumn stages. It's preparing for next season when all will be answered, or at least all that the writers remember to answer.
So in preparation, this season finale has to be the biggest yet, and expectations are high. In short, the fans will revolt if we don't get answers on a few pressing Island mysteries. I've compiled my personal top 5 here. Vote in the poll for your own Island mysteries, and we'll see which ones are solved later tonight.
5. Bram and Ilana

These two characters from Ajira flight 316 could have simply been background dressing. In a classic Lost reversal, we were led to believe that Caesar was the important new character among the crash survivors, an illusion that was quickly shot when he was... well, shot. Since then their image has turned increasingly hostile. They've taken over the Ajira survivors, including Frank Lapidus as their hostage. We also got a glimpse of Bram in 2004, when he kidnapped Miles and urged him not to work for Charles Widmore. He claimed he works for "the side that's going to win." But whose side are they on? Why take control of the Ajira survivors?
Then there's the mysterious question that the characters seem to ask as a test: "what lies in the shadow of the statue?" A nice reference to my #4 mystery.
Chances of Actually Solving It 50%
Bram and Ilana haven't been featured in any of the recent episodes, instead focusing on Locke's new quest and Jack's plot with the hydrogen bomb. Whatever they're up to, we're due to see more of it, but the writers could decide to focus on those two main plots and leave this one for next season.
4. The Statue

The Lost writers love a good tease, and almost no mysteries have been teased as long and vaguely as the giant, four-toed statue. Our first reference to it was way back in season two, when Sayid saw the wreckage of it and commented on its four toes. We didn't get another look until this season, when we saw its back from a distance. Bram and Ilana's question regarding the statue raises questions about both them, and the significance of it to the Island's mythology.
Theories have ranged on the statue's identity. Given the Egyptian motif taking root this season, the most popular theories are that it is a statue of Anubis or Horace. Even if either of those are the case, who built the statue, and for what purpose? Why does it have four toes? And how was it destroyed?
Chances of Actually Solving It 5%
The writers love a good tease, and I have a feeling they're not nearly done with this one yet. It connects to so many other Island mysteries - the Shadow question, Egyptian motifs, the history of the Island - that they may save it for the very end. On the other hand, we're not certain if the statue is whole in 1977, so it could be Jack's hydrogen bomb that cuts it off at the shin.
3. Richard Alpert

Alpert is a fun character; both a source of wisdom and an unquestioning follower, we've been told he works as an adviser of sorts for the Island Natives (aka Hostiles aka Others). We've always had questions about the man. Why doesn't he age? What is his exact role? Why does he wear so much eyeliner? But the latest episode just gave us a new mystery to chew on. When Locke requested to see Jacob, Richard was hesitant. His ensuing discussion with Ben implies that he would rather keep Locke from coming in contact with Jacob, which brings up serious questions about his loyalties to the Island's newly dubbed leader.
Chances of Actually Solving It 75%
Chances are we won't find out all of Richard's mysteries, like his amazing youth. But with Locke dead-set on seeing Jacob, we're certain to at least get more hints on why he was so reluctant in the first place, and why he sees Locke's new mission as a problem.
2. Changing History

Until a few episodes ago, this was apparently impossible. "Whatever happened, happened," said Daniel. It was even an episode title. Now Daniel has presented an alternate theory. The variables, the humans, can change the course of their own destiny. A line cut from the episode, discussed on the official podcast, tells us that tossing pebbles into a stream will only cause ripples, but throwing a boulder can change the course of the entire river. This is the new theory of time travel, and Jack has taken it to heart.
Insistent that he can save everyone some misery, as well as a few lives, Jack wants to enact Daniel's plan to change their own history. Of course, it might kill everyone, which fits nicely with Richard claiming he saw everyone die. We're still not sure Daniel's plan will work, or if it will work as intended. What are the consequences of changing the future? Will the plan work at all, or will it doom everyone as Richard may have predicted? Most of all, what will it mean for Locke and others in present day, who have accepted their place on the Island?
Chances of Actually Solving It 90%
This is a big plot point leading into the finale, so it has to be addressed. A solid portion of the episode will be devoted to this, though it may end on the detonation of the bomb and leave us wondering about the implications of it until next year.
1. Killing Jacob

When Locke dropped this bombshell last week, jaws dropped along with it. Locke is communing with the Island on a deeper level than ever before, and it's led him to make a bold proclamation. The writers haven't given us any hint as to what his intentions are here. Was he speaking literally or figuratively? What is the purpose of this plan? Is it even possible to kill Jacob?
Chances of Actually Solving It 99%
If the hydrogen bomb plot is one big plot point to the finale, this is even bigger. Locke and his followers are currently on their way, and we're eagerly awaiting more. The creators have all but said that this piece of the puzzle will be at least partially filled in during the finale, so we just have to wait until tonight to see it.