Note that this season will be 18 hours, with a two-hour premiere and two-hour finale, aired over 16 consecutive weeks.
6.01/6.02 - "LA X"
6.03 - "What Kate Does" > Kate-centric
6.04 - "The Substitute" > Locke-centric
6.05 - "Lighthouse" > Jack-centric
6.06 - "Sundown" > Sayid-centric
6.07 - "Dr Linus" > Ben-centric
6.08 - "Recon" > rumored to be Sawyer-centric
6.09 - "AB Aererno" > Richard-centric
6.10 - (Title unknown at this time) > Jin/Sun-centric
6.11 - "Happily Ever After" > Desmond-centric
Titles for episodes from 6.12 to the finale, as well as whom they will center around, have not been leaked yet.

Characters coming back
The regular characters this season will be: Sayid, Richard Alpert, Claire, Ben, Frank Lapidus, Jack, Hurley, Sawyer, Jin, Sun, Miles, Kate, Locke and Ilana.
Michael, he will appear in the second half of the season
Libby, she will appear in the second half of the season
Rose
Juliet
Charlie will be in four episodes
Boone
Charlotte
Eloise
Widmore
Christian
Helen, she'll appear in the first third of the season
The producers are trying to find ways to have Walt back, but the fact that the actor aged and looks much older now is making it tricky.
They are trying to get Shannon back, but Maggie Grace has a really busy schedule.
Hurley's fortune teller, who was played by Suzanne Krull, will be back. Rumors say she may pop up in episode three.
And much more!

Storylines
We'll see who the characters used to be and who they are now. Some reports say that it will be an "on island" and "alternate reality" type of format this season. So keep that in mind when reading the spoilers below.
The season premiere picks up right after the finale, and a large part of it will take place on Flight 815. Desmond will be on this flight. The Statue will be seen from the plane. Jack will bump into Kate on the plane.
Oceanic Flight 815 pilot Seth Norris, played by Greg Grunberg, will be back for the season premiere in a voice over.
Shannon's absence from the beginning of the new season will be explained by a significant change to Shannon and Boone's backstory.
One Lostie we've known for a long time (a dark, handsome someone) will die within the first four hours in the first shocking death of the year. Could this be Rose or Sayid? Two spoilers (see below) hint that they could die.
One of the following is true: Sayid dies. Sayid kills Jack.
Kate will beat up the Marshall.
Juliet will die, but not before a final kiss.
Jacob will appear to Hurley.
Sawyer and Jack will fight again.
Another Lostie we've known and loved since day one of season one will come thisclose to dying at his own hands -- but the good doctor (Matthew Fox's Jack Shephard) will save him. Could this be Charlie? As there is another spoiler saying that Jack will save Charlie again.
All of these things will be said in the first six hours, and they are pivotal to the plot: "Am I alive?" "It turned out she did not want out." "I wanted you to be able to go home." "I was killed by an *** ******." "I was supposed to die." The first one is not a female, and is the same person as #5.
In episode 6.04, we will see that Hurley and his Hummer will cause a problem for Locke; Rose has a connection to Hurley, which is used to help Locke; Rose is not long for "this" world; Locke will run into an old enemy.
Jack and Hurley will discover a special mirror in a lighthouse that, when aligned properly, gives them a flash of their past.
In episode 6.11, Eloise will host a big fundraiser that Desmond crashes to find Penny. Room 23 and Widmore will be in the episode.
Kate or Jin will experience something called "Room 23."
Six of these things are true: Juliet survives the bomb; Juliet does not survive the bomb; Kate and Sawyer flirt; Kate and Sawyer kiss; Juliet and Sawyer kiss; Jack and Sayid touch lips; there is a scene with Sawyer in the season premiere that will make you cry and/or make Sawyer/Kate shippers happy.
There seems to be more happening between Sawyer and Kate than Jack and Kate in the beginning of the new season.
Claire will threaten to kill Kate.
We may get our Libby questions answered.
Three main characters will be apprehended separately by the authorities this season.
A dead character also may "return" without us actually seeing the actor who plays him or her! Miles will be carrying an important message from a deceased character.
One of the following things is true regarding Ilana and her Ajira posse: They were sent by Charles Widmore to kill Ben. They were sent by the Man in Black (Jacob's nemesis) to find the island's time travel mechanism and destroy it. They were sent to protect Jacob. They were sent to revive John Locke. Ilana is future Sayid in drag.

The numbers

A list will be found where the characters have been assigned certain numbers. Below are only the names of the people who were assigned a number that is in the show's famous number sequence.
4 = Locke
8 = Hurley
15 = Sawyer
16 = Sayid
23 = Jack
42 = Kwon (Sun or Jin)

As I've said above, this isn't a complete list of all the spoilers released about the final season so far. Some can also be found in past Spoilers Anonymous. However, if you have spoilers for the final season of Lost that haven't been posted in this post, feel free to share them with us in comments below!