Back in the future, other minor questions are addressed. It's now clear how Ben was injured (as seen in "The Life and Death of Jeremy Bentham"), and it was confirmed that Frank and Sun were the "pilot and woman" who left with one of the outriggers. The round figure of "thirty years earlier" works if the Ajira Tribe was on Hydra Island in 2008.
The appearance of Christian Shephard was a surprise, but it does fit his more active role of late. Christian was involved in whatever happened with Claire, in getting Ben to move the island, and getting Locke to repair Ben's mistake. Now he seems to be setting Frank and Sun on a new adventure, presumably back into the past. (Of course, that's supposition, but it fits the theme of the season thus far.) Whether he's an agent for Jacob or Jacob himself in the flesh of Christian Shephard, it's a sign that Jacob is taking an active role in the fate of the island.
This is essentially the mid-point of the season, so this is designed to set the stage for the second half and start building the momentum towards the finale. The current arc promises to tie up even more loose ends, and it's always good to get more clarification on the Dharma Initiative.