So I feel like throwing out some of the theories I gathered after watching the episode:
Obviously, OT Desmond remembered his trip to the FST. And did you guys catch that the beginning of that trip was to when he's looking at his reflection? And I think he's sent back to the OT because he makes contact with his Constant.
Near-death experiences make you see into the timeline you're not in: such as Juliet seeing into the FST in LA X, and Charlie seeing Claire this episode. And I suppose if there's a strong enough (possibly romantic) connection between characters they experience bleedover- Penny and Desmond, Dan and Charlotte, possibly Jack/Kate/Sawyer.
And did anyone notice that basically every character significant to Desmond popped up this episode? Charlie, Widmore, Penny, Faraday, Eloise, even Minkowski.
And leave it to Desmond to make the only FS that you actually care about. By far the best FS we've had. This is probably my favorite episode of the season because of it making the FST better and exploring its purpose. What are everyone's thoughts?