I was happy to see them get moving relatively full steam ahead. I'm kind of sad that the end is this close, but they are abviously going to keep us interested, confused, thinking and guessing right up to the end.
My wife suggested that Jacob has assumed control of Sayid's body. I'm not sure about that--it would be something new for the show, as MIB takes on the appearance but doesn't actually animate or inhabit the corpse itself. I'm leaning away from that for the time being.
I believe that Juliet was experiencing a flash into the alternate timeline--just like Desmond did after a similar experience with the power under the Swan station. She was confused and phasing in and out of reality like we saw Desmond (and later, Charlotte) do, hence her talk of coffee, and was experiencing her life off the island in the alt during her silent moments. That would explain the whole "It worked" comment. She had witnessed/visited the reset timeline.
However...
I think that Jack's sense of deja vu/disorientation--and particularly the cut on his neck--are important clues that something more than a simple reset is going on. They obviously want us wondering about that cut--how and when Jack got it. It's possible that they are tricking us into thinking/assuming that the alternate timeline was a result of the incident/Jughead detonation. Maybe the alternate timeline was created by climactic events on the island that we haven't seen yet--the season ends with the island going way back in time and sinking to the bottom of the sea when the volcano erupts, and Jack flashes forward into the future of that timeline with some sort of mission to accomplish. That would make the events in the alternate timeline follow after the on-island events of season six, although we would be watching them unfold parallel to each other over the next four months as they both reach a climax in the series finale--the island storyline finishing with Jack (cutting his neck) and then flashing back to start the loop over again (something many have suspected to be the end of the show), but instead of flashing back to the Pilot episode, he flashes to the alternate timeline we saw last night, and follows it to a new conclusion with an actual ending and saves the day. Somehow.
They obviously have to tie the alt story into the one we've been following for five years--I'm just trying to figure out how that's going to happen. If the purpose of the alt timeline is to show the continuation of Jack's journey in an attempt to save or preserve everything we've seen before, then we would be emotionally invested in it. So I'm hoping that the alt story actually takes place after the events we are currently watching unfold on the island, and the alt flashes are actually showing us the end of the series. Jack's purpose in the alt reality is to prepare the way and preserve some sort of chain of events so that things will be ready when everybody on the island dies in the series finale and their minds flash over into their bodies in the alt. (Something which apparently happened to Juliet.) Death and sacrifice on the island in the finale will be rewarded with a happily ever after future in the restored alternate timeline (which you might even view as the original timeline before Jacob/MIB/Widmore/Richard started meddling with everyone's lives).
This is all just random rambling based on only one week's worth of new information, but there really does need to be some sort of connection between the two storylines that lead to a satisfying conclusion, and I can't wait to see what the writers have up their sleeves.