Locke saying what he saw was beautiful when looking into the eye of the island to Mr. Eko / Jack. Maybe he seen the image of this light in a tunnel
some questions i feel we won't find out are;
Where 'mother' came from
Where the real Jacob/Adam's mother came from
although i do think we'll find out what the Smoke Monster is in more depth, and maybe what Adams real name is.
Then we've got the whole War going on with Flocke(Adam/Smokie/MiB) and Widmore, the flash sideways to follow, and the whole conclusion to something that has been in our lives for 8 years or whatever, in 3 episodes.
I suppose there is still quite a bit
Don't know why they just didn't expand the season out unless there was production issues, have a feeling some people are going to be disappointed whatever happens though.